Theoretical Fundamentals of Atmospheric Optics 1904602258, 9781904602255

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THEORETICAL FUNDAMENTALS OF ATMOSPHERIC OPTICS

THEORETICAL FUNDAMENTALS OF ATMOSPHERIC OPTICS Yu.M. Timofeyev and A.V. Vasi'lev

CAMBRIDGE INTERNATIONAL SCIENCE PUBLISHING

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ISBN 978-1-904602-25-5 Cover design Terry Callanan Printed and bound in the UK by Lightning Source Ltd

Contents PREFACE ...........................................................................................ix 1. THE SOLAR SYSTEM: PLANETS AND THE SUN ...................1 1.1. 1.2. 1.3. 1.4.

The planets of the solar system ............................................................ 1 Main parameters of the atmosphere of the planets .............................. 2 Special features of the orbit of the Earth ............................................. 8 The Sun and its radiation .................................................................... 13

2. ATMOSPHERE OF THE EARTH ...............................................24 2.1. 2.2. 2.3. 2.4. 2.5.

Division of the atmosphere into layers ................................................ 24 Spatial and time variability of the structural parameters of the atmosphere ......................................................................................... 29 Gas composition of the atmosphere .................................................... 34 Atmospheric aerosol ........................................................................... 42 Clouds and precipitation ...................................................................... 49

3. PROPAGATION OF RADIATION IN THE ATMOSPHERE ...53 3.1. 3.2 3.3. 3.4. 3.5. 3.6.

Electromagnetic waves ...................................................................... 53 Intensity and radiation flux .................................................................. 57 Characteristics of interaction of radiation with a medium .................. 64 Radiation transfer equation ................................................................. 76 Complex refraction index. Polarisation of radiation. Stokes parameters .............................................................................. 93 Radiative transfer equation taking polarisation into account ............. 104

4. MOLECULAR ABSORPTION IN THE ATMOSPHERE ....... 114 4.1. 4.2. 4.3.

The general characteristic of molecular absorption in the atmosphere of the Earth ................................................................... 114 Different types of molecular absorption ........................................... 118 Absorption spectra of atmospheric gases ......................................... 123 v

Theoretical Fundamentals of Atmospheric Optics

4.4. 4.5. 4.6. 4.7.

Quantitative description of molecular absorption .............................. 124 The shape of spectral absorption lines .............................................. 137 Quantitative characteristics of molecular absorption ........................ 155 Molecular absorption in the Earth atmosphere ................................. 164

5. LIGHT SCATTERING IN THE ATMOSPHERE .....................170 5.1. 5.2. 5.3. 5.4. 5.5. 5.6.

Molecular scattering ......................................................................... 170 Scattering and absorption on aerosol particles .................................. 183 Aerosol scattering and absorption in the atmosphere ....................... 202 Scattering of radiation with redistribution in respect of frequency ... 216 Atmospheric refraction ..................................................................... 224 Optical phenomena in the atmosphere .............................................. 237

6. OPTICAL PROPERTIES OF UNDERLYING SURFACES .....250 6.1. 6.2. 6.3. 6.4. 6.5.

Main special features of reflection of radiation ................................ 250 Quantitative characteristic of reflection of radiation (mirror reflection) ............................................................................. 253 Quantitative characteristics of reflection of radiation (real surfaces) .................................................................................. 258 Examples of the optical characteristics of underlying surfaces ........ 264 Emitting properties of underlying surfaces ....................................... 274

7. FUNDAMENTALS OF THE THEORY OF TRANSFER OF ATMOSPHERIC RADIATION .............................................279 7.1. 7.2. 7.3. 7.4. 7.5. 7.6. 7.7.

Transfer of thermal radiation ............................................................ 280 Transmittance functions of atmospheric gases ................................. 283 Methods of determination of transmittance functions ...................... 286 Approximate methods of radiation transfer theory ........................... 305 Thermal radiation fluxes ................................................................... 315 Non-equilibrium infrared radiation .................................................... 323 Glow of the atmosphere ................................................................... 326

8. MAIN CONCEPTS OF THE THEORY OF SOLAR RADIATION TRANSFER .................................................................335 8.1. 8.2. 8.3. 8.4.

Multiple scattering of radiation ......................................................... 335 Analytical methods in radiation transfer theory ................................ 344 Numerical methods in the theory of radiation transfer ..................... 356 Algorithms and programmes for calculating radiation characteristics of the atmosphere (radiation codes) ......................... 370 vi

Contents

9. RADIATION ENERGETICS OF THE ATMOSPHERE– UNDERLYING SURFACE SYSTEM ..................................375 9.1. 9.2. 9.3. 9.4. 9.5.

Solar insolation at the upper boundary of the atmosphere ................ 375 Radiation balance of the surface ...................................................... 377 Radiation balance of the atmosphere ............................................... 382 Radiation balance of the planet ........................................................ 392 Radiation factors of climate changes ............................................... 400

10. RADIATION AS A SOURCE OF INFORMATION ON THE OPTICAL AND PHYSICAL PARAMETERS OF ATMOSPHERES OF PLANETS .........................................................407 10.1. Direct and inverse problems of the theory of transfer of radiation and atmospheric optics ..................................................................... 407 10.2. Remote measurement methods ........................................................ 410 10.3. Classifications of remote measurement methods ............................. 414 10.4. Remote methods of measurement based on measurements of attenuation (absorption) of radiation ................................................. 417 10.5. Remote methods using measurements of atmospheric radiation ...... 427 10.6. Remote measurement methods based on recording the scattered and reflected solar radiation ............................................................. 439 10.7. Active remote measurements methods ............................................ 443

APPENDIX. FUNDAMENTAL UNITS IN ATMOSPHERIC OPTICS AND PHYSICS ..............................................................447 A.1. Molecular mass of dry and moist air ................................................ 447 A.2. Units of measurement of temperature, air pressure and gas composition of the atmosphere ......................................................... 451 A.3. Units of measurement of the concentration of water vapour ........... 457 A.4. Gas content and units of measurement ............................................ 461 A.5. Units of measurement of spectral intensities in radiation fluxes Planck formula in different units ....................................................... 463 A.6. Units of measurement of the coefficients of molecular scattering and absorption .................................................................................. 466 A.7. Units of measurement of the concentration of aerosols and volume coefficients of aerosol extinction ......................................... 467 References ............................................................................................... 471 Index ......................................................................................................... 477

vii

Preface Various atmospheric phenomena have been of interest to man from time immemorial. The life of people and other representatives of the fauna and flora has depended greatly on the weather and climate on our planet and on illumination conditions. The dependence of the mankind on the weather and climate is still very considerable. Fluctuations of precipitation, anomalous temperatures and winds have the controlling effect on the life of people. Long-term droughts result in the deaths of tens and hundreds of thousands of people, regardless of the help provided by various international and charitable organisations and funds. Tens and hundreds of people die during flooding, cyclones and storms. One of the books written by Aristotle is titled ‘Meteorology’ and is dedicated in particular to the description and attempts to explain various atmospheric phenomena. For example, a relatively rational explanation of phenomena such as halo and rainbow is proposed. The subjects, discussed in ‘Meteorology’, are now the subjects of various sciences, not only meteorology. The Meteorology book is interesting not only as an attempt to explain natural phenomena but also as experience with the application of unified principles for explaining different phenomena. The book may be regarded as the first book on meteorology available to us. The accurate publishing date is not known, but it may be assumed that it was written more than 2300 years ago. Many optics laws were discovered a long time ago. For example, the law of direct propagation of light is found in a report on optics attributed to Euclid (300 years BCE) and it is likely that this phenomenon had been known long time prior to this date. The law of light reflection is also mentioned in the book Optics by Euclid. The phenomenon of light refraction was already known to Aristotle (400 years BCE). The sources of current atmospheric optics are found in optical sciences which have been formed to a large extent on the basis of observation of natural optical phenomena. Studies of various atmospheric optical phenomena were carried out by scientists such as Newton, Foucault, Euler, Roemer, Huygens, Lomonosov and many others. Current atmospheric optics includes information on the physical ix

Theoretical Fundamentals of Atmospheric Optics

state of the planet atmosphere, various sections of classic optics, radiation transfer theory, atomic and molecular spectroscopy, and electrodynamics. If main attention in the initial stages of development of atmospheric optics was paid to investigations in the theory of visibility and radiation energetics of the atmosphere, then at the present time atmospheric optics studies and describes greatly differing optical phenomena, both from the energetics viewpoint and from the viewpoint of angular, spectral and temporal dependences of the characteristics of the radiation field and of the factors which determine these characteristics. A powerful impetus for the further development of atmospheric optics has been the need to perform remote measurements (ground-based, space) of different atmospheric parameters and of the surface. To realise these methods, it is necessary to achieve even deeper understanding of various processes of interaction of radiation with the medium (atmosphere and surface) and even higher accuracy of definition of the quantitative parameters of the interaction. Atmospheric optics is part of the atmospheric physics – the science of physical processes in the Earth atmosphere (and also other planets). Atmospheric physics includes theoretical description and experimental investigations of all atmospheric phenomena. Atmospheric physics and, in particular, atmospheric optics are connected with greatly differing scientific disciplines, because atmospheric processes influence almost all aspects of the life of the mankind. A suitable example confirming this is the connection of atmospheric optics with medicine: the number of skin cancer patients is directly connected with the amount of ultraviolet radiation of the Sun reaching the Earth surface. The interests of the current atmospheric optics include: – the processes of transformation of radiation energy of the Sun in the atmosphere and on the surface, and the formation of different types of atmospheric radiation as components of the radiation balance of the planet; – the processes of propagation and transformation of different types of radiation (solar, thermal, non-equilibrium) which determine the temporal, spatial, polarisation and other characteristics of radiation fields, in particular, illumination of the surface; – the radiation field as a source of information on the optical and physical characteristics of the atmosphere and the surface. This book has been written on the basis of long-term experience of reading lectures and seminar courses by the authors at the Department of Atmospheric Physics of the Faculty of Physics of the St. Petersburg State University. x

Preface

The first chapter is concerned with a brief examination of the main special features of the structure of the solar system. Brief information on the planets of the solar system is provided. The main characteristics of Earth motion and its changes are discussed. The structure of the Sun and special features of formation of solar radiation are investigated. Data are presented on the variability of solar energy reaching the Earth and on the spectral distribution of solar radiation. The second chapter discusses essential general information on the Earth atmosphere. Up-to-date information on the division of the atmosphere into layers, based on different criteria, is provided. The spatial and temporal variations of the main structural parameters of the atmosphere – temperature, density and pressure – are discussed. Various forms of the equation of state of the gas and the statics equations, barometric relationships, and application ranges for these relationships are outlined. Special attention is paid to the characteristics of the gas composition of the atmosphere, temporal variability and longterm variations caused by various antropogenic factors. The data on atmospheric aerosols, clouds and precipitation are briefly analysed. Various characteristics of these parameters – the concentration of particles, the size distribution function of the particles and intensity of precipitation are introduced. The third chapter discusses the definitions of the main optical characteristics – intensity, radiation flux, – consideration of the division of the electromagnetic spectrum into individual ranges. The main processes of interaction of radiation with the gas–aerosol medium are analysed. The radiation transfer equation in different forms is introduced. The polarisation characteristics of radiation and the vector form of the transfer equation are presented. The fourth chapter discusses in detail molecular absorption in the atmosphere and its main special features. Various types of molecular absorption and parameters describing this process are analysed. Attention is also given to the characteristics of selective molecular absorption, such as the position, intensity, half-width of spectral lines, and their dependence on the parameters of the physical state of the medium. The main mechanisms of formation of the absorption line shapes – natural broadening, broadening as a result of collisions, and the Doppler effect, – are discussed. The quantum-mechanics form of the transfer equation is presented, and the equation is used to derive expressions for the characteristics of selective molecular absorption. The fifth chapter describes various types of scattering in the Earth atmosphere. The coefficient and phase function of the molecular scattering of radiation are derived. The polarisation characteristics of xi

Theoretical Fundamentals of Atmospheric Optics

molecular scattering are analysed. Scattering both on individual aerosol particles and aerosol scattering in the atmosphere as a whole are investigated. Various optical phenomena, connected with aerosol scattering, are studied. Various types of non-coherent scattering in the atmosphere and its significance in solving atmospheric physics problems are dealt with. The optical phenomena, determined by radiation refraction, are investigated and many atmospheric optics phenomena are explained. The sixth chapter introduces and analyses various optical characteristics of the reflecting properties of natural surfaces. Surface albedo, the spectral brightness coefficient and reflection factor are defined, and the relationships between various reflection characteristics are presented. A large number of examples of the optical characteristics of the underlying surface in different ranges of the spectrum are given. The seventh chapter is concerned with consideration of various aspects of the theory of atmospheric radiation transfer. The case of the transfer of thermal radiation for a plane-parallel model of the atmosphere is investigated in detail. Various methods of deriving the transmittance functions are outlined. Information is provided on various modelling approaches in calculations of the transmittance function and the approximate methods, used in the calculations of the transmittance functions and characteristics of thermal radiation, are analysed. Information on the non-equilibrium infrared radiation of the Earth atmosphere is provided. Chapter 8 describes the fundamentals of the theory of solar radiation transfer. Information is given on various methods of solving the radiation transfer equation with multiple light scattering taken into account. The nature and main special features of various types of atmospheric glow are investigated. Special features of the formation of the ozone layer of the Earth and various photochemical processes in the Earth atmosphere are also briefly described. In chapter 9, attention is given to the study of the problems of radiation energetics of the atmosphere. The relationships for calculating the solar energy flux at the upper boundary of the atmosphere at different moments of time and at different latitudes are presented. The radiation balance of the underlying surface, the atmosphere and of the planet as a whole is discussed. The tenth chapter deals with direct and inverse problems of radiation transfer theory and atmospheric optics. Special attention is given to various remote measurement methods. The remote methods of measuring the parameters of the atmosphere and the Earth surface are xii

Preface

classified, examples of various passive remote measurement methods, using the measurement of the radiation, passing through the investigated medium, and radiation of the atmosphere–underlying surface system are given. Active remote measurement methods are briefly characterised. The appendix contains information on the main measurement units of atmospheric and optical parameters. The authors are grateful to their colleagues, G.I. Gorchakov, D.I. Naringer and M.V. Tonkov, who reviewed the individual chapters of the book and made a number of important comments. They are especially grateful to G.M. Shved who made many useful critical comments and constructive proposals which greatly increased the quality of presentation of the material. Finally, the authors would like to express their gratitude to colleagues of the Department of Atmospheric Physics of the Moscow State University (department head V.E. Kunitsyn) who edited the book. The book could not be written without the help of colleagues of the Department of Atmospheric Physics of the Research Institute of Physics of the St. Petersburg State University, E.M. Shulgina, T.A. Naumova and L.N. Poberovskaya, who worked very hard on the preparation of the book for publication.

xiii

CHAPTER 1

THE SOLAR SYSTEM: PLANETS AND THE SUN 1.1. The planets of the solar system In examination of the physics and properties of the atmosphere, it is useful to mention the main facts of the solar system, the Earth as a planet, its size and shape, special features of its orbit, the Sun and its radiation, and other planets. This is also important because the concepts such as ‘atmospheric physics’ and ‘atmospheric optics’ are applicable not only to the Earth but also to any other planet with an atmosphere. The planets The Earth is one of the nine planets of our solar system [16, 50, 94]. Table 1.1 gives the main astronomical parameters of the planets. Table 1.1 Main astronomical parameters of planets P la ne t Me rc ury Ve nus Ea rth Ma rs Jup ite r S a turn Ura nus N e p tune P luto

R, a u

r, k m

T

t

ϕ

g , units o f g 0

A

0 387 0 723 1 000 1 524 5 203 9 555 19 22 3 0 11 39 44

2439 6051 6378 3379 71300 60100 24500 25100 1 5 0 0 (? )

88 d 225 d 365 d 687 d 11 8 6 y 29 46 y 84 01 y 164 8 y 247 7 y

59 d –243 d 23 h 56' 24 h 37' 9 h 50' 10 h 14' 17 h 17' 16 h 7' 6d9h

0º 3º 23º 27' 25º 12' 3º 26º 44' 98º 28º 48' ?

0 37 0 88 1 00 0 38 2 64 1 15 1 17 1 18 0 0 4 6 (? )

0 056 0 72 0 29 0 16 0 343 0 342 0 34 0 29 04

C o mme nt: R – The me a n d ista nc e o f a p la ne t fro m the sun in a stro no mic units (1 a u = 1 4 9 5 0 0 0 0 0 k m – the me a n d ista nc e fro m the Ea rth to the sun); r – the ra d ius o f the p la ne t; T – the p e rio d o f ro ta tio n o f the p la ne t a ro und the sun (in d – d a ys, y – ye a rs); t – p e rio d o f ro ta tio n a ro und its a xis (the minus sign ind ic a te s ro ta tio n in the d ire c tio n o p p o site to tha t o f the Ea rth);ϕ 1 is the a ngle b e twe e n the p e rp e nd ic ula r to the p la ne o f the e lip tic a nd a xis o f ro ta tio n o f the p la ne t;1 g – the ro ta tio na l a c c e le ra tio n o f the surfa c e o f the p la ne t e xp re s se d in the units o f a c c e e le ra tio n o n the surfa c e o f the Ea rth g0 = 9 8 1 m s- 1; A is the inte gra l a lb e d o o f the p la ne t – fra c tio n o f the re fle c te d e ne rgy o f the sun a rriving o n the p la ne t 1

The p la ne o f the e c lip tic – the p la ne in whic h the Ea rth ro ta te s a ro und the sun

1

Theoretical Fundamentals of Atmospheric Optics

The data presented in Table 1.1 clearly indicate large differences of the planets of the solar system in respect of different characteristics. In the following paragraph, it will be shown that the atmospheres of the planets also greatly differ. This fact is of considerable research and scientific value. The nature has presented to the man unique possibilities of examining very ‘contrasting’ planets. If it is taken into account that the possibilities of organisation of directional or even full-scale experiments – the main method of examining the physical processes – in the physics of the atmosphere ae greatly restricted, then the examination of, in particular, atmospheres of different planets offers a unique possibility of verifying the physical models, for example, the general circulation of the atmosphere. Finally, when discussing the planets, it is also important to mention their satellites. This is also important because of the fact that the Earth has a single and very large satellite and reflection of solar radiation from the satellite during the night plays a significant role in the formation of the radiation field in the atmosphere of the Earth (although the radiation from the Moon at the upper boundary of the atmosphere of the Earth is almost 6 orders of magnitude lower than the radiation from the Sun). Mars has two satellites (Phobos and Deimos). The giant planets have many satellites. The largest satellites, with the size only slightly smaller than that of Mercury, are the sixth satellite of Saturn – Titan and the third satellite of Jupiter, Ganymede. Some satellites, for example, Titan, have relatively thick atmospheres.

1.2. Main parameters of the atmosphere of the planets Main characteristics of planet atmospheres All the planets of the solar system have atmospheres [50, 94]. Mercury and Pluto have very rarefied atmospheres and, to a certain approximation, it may be assumed that these planets have no atmosphere. The atmospheres of the planets, like the planets themselves, greatly differ. Table 1.2 shows the data on the gas composition of the atmospheres of the planets and satellites, and the range of gases corresponds to the order of decreasing concentration. Table 1.3 gives the main physical parameters of the atmospheres of the planets (with the exception of Pluto for which only a few reliable data are available). It should be mentioned that the physical parameters of the atmospheres are linked by the relationships well2

The Solar System: Planets and the Sun Table 1.2. Gas composition of the atmosphere of planets and satellites [94] P la ne t

Ma in c o mp o sitio n

Me rc ury Ve nus Ea rth Ma rs Jup ite r S a turn Ura nus N e p tune P luto Io Tita n Trito n

CO2 CO2 N 2, O 2 CO2 H2 , He H2 , He H2 , He H2 , He C H4 SO 2 N2 N2

S ma ll ga s c o mp o ne nts

Ar, H2O , C O , HC l, HF r Ar, H2O , C O 2, C H4, O 3 N O a nd o the rs N 2 , O 2 , C O , H2 O , H2 , O 3 , N O N H3, P H3, C H4, C 2H6, C 2H4, a nd o the rs N H3 , P H3 , C H4 , C 2 H6 , C 2 H2 , C O C H4 C H4 , C 2 H2 , C 2 H4 , C 2 H6 H2 , He Ar, H2, C H4, C 2H2, C 2H4 C H4

Table 1.3. Main physical parameters of planet atmospheres [94]

P la ne t

M, g/c m2

µ, g/mo l

c p, J/g· K

κ

γ a, K /k m

v s, m/s

T ee, K

Me rc ury Ve nus Ea rth Ma rs Jup ite r S a turn Ura nus N e p tune

100 µm, it is important for remote (satellite) sounding of the atmosphere and the surface of the Earth. The main absorbing agents in this region are O 2 and H 2 O (Fig. 4.16). Remote measurements of the temperature profile are taken using the band of O 2 at λ = 0.5 cm (60 GHz) and the separate absorption band λ = 0.25 cm (120 GHz). The absorption bands of H 2 O, situated in the vicinity of λ = 1.35 cm (22.2 GHz) and λ = 0.16 cm (187.5 GHz) are suitable for remote measurements of the profile of water vapours. In the microwave region of the spectrum there is also a large number of rotational lines of many atmospheric gases [109], for example, ozone, ClO, NO, NO 2, HNO 3, etc. Although their intensity in the absorption spectrum of the Earth atmosphere is relatively low, at a specific geometry of measurements and in the presence of highsensitivity devices they are used for the remote measurements of the characteristics of the gas composition of the atmosphere. 168

Molecular Absorption in the Atmosphere

ν,GHz Fig. 4.16. Optical thickness of the atmosphere τ as a function of frequency ν in the microwave range. Cross-hatched area – absorption by H2O not taken into account, upper curve – absorption by H 2O taken into account [119]

169

CHAPTER 5

LIGHT SCATTERING IN THE ATMOSPHERE 5.1. Molecular scattering Modern optics uses two physical models, explaining the nature of scattering of light by gases: the scattering of light directly on molecules (atoms) of the gases (Rayleigh–Tindall theory), and the scattering of light on thermal fluctuations of the density of the gas, resulting in identical fluctuations of its refractive index (Einstein– Smolukovskii theory). The fluctuations theory of scattering is more general, and the case of molecular theory is regarded as a partial one. However, in atmospheric optics, by tradition ascending from Rayleigh, it is recommended to use the molecular theory of scattering [20, 24, 26, 32, 33, 37] and, for this reason, we shall also use his theory.

Derivation of equations for molecular scattering We examine the interaction of an electromagnetic wave with air molecules. It is assumed that the electromagnetic wave is incident on a separate molecule. Since the dimensions of the molecule are considerably smaller than the wavelength, all points of the molecule will be found in the electromagnetic field of the same strength since the spatial variations of the strength at distances equal to the size of the molecule are ignored. Thus, the external field, acting on the molecule, may be regarded as homogeneous. Under the effect of the electrical field of the incident wave, the charges of the particles, forming the molecule (the phenomenon of polarisability of matter) are separated, and the molecule acquires its own electrical field. This field is approximated as the field of the electric dipole. The oscillations of the external field (with time) lead to identical oscillations of the dipole, i.e. its movement with acceleration and, consequently, the dipole itself becomes the secondary centre of generation of the electromagnetic wave. It is a secondary wave and radiation is scattered. It is also assumed that an external field with the strength E 0 is 170

Light Scattering in the Atmosphere

incident on the molecule and induces the dipole moment of the molecule P. At the start, it is assumed that the incident wave is characterised by linear polarisation (chapter 3). Consequently, vectors E 0 and P are always found in the same plane. The directions of oscillations of the vectors are parallel. We use the formula, available in electrodynamics, for the field of a vibrating dipole E 1 in the long-range zone (i.e. at r >> λ):

E1 (θ) =

1 ∂2 P sin θ, 2c 2 r ∂t 2

(5.1.1)

where θ is the angle between the axis of the dipole and the direction of scattered radiation; r is the distance from the dipole to the observation point. We take into account the relationship of the dipole moment with the external field: P = α∼ E 0 ,

here α is the polarisability of the medium (the gas in the present case). Consequently, remembering (3.1.1):

E0 ( x, t ) = E0,0 cos(2πνt − E1 = −α

2π x + δ), λ

1 (2πν)2 E0 sin θ. 2 cr

(5.1.2)

In final analysis, we are interested in the intensity of scattered radiation. In chapter 3 it has been confirmed that the intensity of radiation in vacuum is independent of distance. Consequently, the intensity of scattered radiation is independent of distance both directly or indirectly (through constants which depend on r). Therefore, we are completely justified to ignore, already in the initial stage, the dependence of the investigated quantities on r without waiting until r ‘cancels’, and write (5.1.2) in the form:

E1 = −α

1 (2πν) 2 E0 sin θ. c2

(5.1.3)

It is now assumed that the incident wave is characterised by elliptical polarisation in a general case. Consequently, as shown in chapter 3, the vector of the electrical field of the wave maybe decomposed into two mutually normal components. We select (Fig. 5.1) the component E 0,|| situated in the plane formed by the 171

Theoretical Fundamentals of Atmospheric Optics

direction of the incident wave E 0 and the scattered wave E 1 (γ). This plane is referred to as the scattering plane. Component E 0,⊥ is normal to the scattering plane. However, in this case for E 0⊥ , the angle between the component and the scattering direction is always π/2 and sin θ = 1. It is also taken into account that the scattering angle γ is the angle between the directions of the incident and scattered radiation, i.e. (Fig. 5.1) the angle γ = π/2–θ, and consequently, sinθ = cos γ. Taking this into account, from (5.1.3) we obtain:

E1, = −α

1 (2πν) 2 E0, cos γ, c2

1 E1, = −α 2 (2πν ) 2 E0,⊥ . c

(5.1.4)

The resultant equations (5.1.4) are equations (3.6.3) for the relationship between the electrical vectors of the incident and scattered waves. Consequently, the coefficients of this relationship are:

S1 = −α

1 1 (2πν )2 cos γ , S 4 = −α 2 (2πν) 2 . 2 c c

(5.1.5)

The matrix and phase function of molecular scattering According to (3.6.5), we obtain the elements of the matrix of molecular scattering

Fig. 5.1. Scattering of an electromagnetic wave on an air molecule.

172

Light Scattering in the Atmosphere

1 D11 ( γ ) = (S1 S1 + S4 S4 ) = A(1 + cos2 γ ), 2 1 D12 ( γ ) = (S1 S1 + S4 S4 ) = A(cos2 γ − 1) = − A sin 2 γ ), 2 1 D33 ( γ ) = (S1 S4 + S1 S4 ) = 2 A cos γ, 2

(5.1.6)

i D34 ( γ ) = (S1 S4 − S1 S4 ) = 0, 2 where (taking into account that the wavelength of light in vacuum is λ = c/ν)

A=

α 2 8π4 ν 4 α 2 8 π4 . = c4 λ4

(5.1.7)

Now, we normalise the scattering matrix, and for this purpose it should be remembered that it is necessary to separate all its elements to the integral

∫

∫ π

1 1 D11 d Ω = D11 γ sin γd γ, 4π 4 π 20

which, after calculations, gives

∫

∫

π

1 1 4 A(1 + cos2 γ )sin γd γ = A (1 + x 2 )dx = A. 20 2 −1 3 1

(5.1.8)

Thus, we have obtained the normalised matrix of molecular scattering 0 0   1 + cos2 γ − sin 2 γ   2 2 0 0  3  − sin γ 1 + cos γ , 4 0 0 2 cos γ 0    0 0 0 2 cos γ  

(5.1.9)

In the first element of this matrix is the phase function of molecular scattering

173

Theoretical Fundamentals of Atmospheric Optics

3 x( γ ) = (1 + cos2 γ ). 4

(5.1.10)

According to (5.1.10), molecular scattering is not isotropic. It is greater in the forward and backward directions, and smaller in the lateral direction. The phase function (5.1.0) is the Rayleigh scattering phase function (Fig. 5.2), and molecular scattering is often referred to as Rayleigh scattering.

The cross-section and volume coefficient of molecular scattering We return to the value of the normalisation potential (5.1.8). In chapter 3 (paragraph 3.6), it was explained that the scattering matrix of a particle (a molecule in the present case) is represented in the form of the product of the scattering section and the normalised scattering matrix, i.e. the relationship (3.6.7). Consequently, the normalisation coefficient (5.1.8) is nothing else but

4 1 A = Cs , 3 4π Here C s is the cross-section of molecular scattering and

4 128π5 α 2 Cs = 4 π A = . 3 3λ 4

(5.1.11)

The expression for the polarisability of a homogeneous dielectric in a homogeneous field, known from electrostatics, is the following:

α=

ε − 1 n2 − 1 = , 4 πN 4 πN

(5.1.12)

Fig. 5.2. Molecular scattering phase function (Rayleigh phase function).

174

Light Scattering in the Atmosphere

where ε = n 2 is dielectric permittivity, associated with the refractive index of the gas n; N is the number of gas molecules in the unit volume, i.e. the countable concentration of the gas. Finally, we obtain the following equation for the molecular scattering crosssection

Cs =

8π3 ( n2 − 1)2 . 3N λ 4

(5.1.13)

Assuming that all molecules interact independently with the radiation, we obtain the following equation for the volume coefficient of molecular scattering σ = NC s

σ=

8π3 (n2 − 1)2 . 3N λ 4

(5.1.14)

The relationships (5.1.13) and (5.1.14) are slightly confusing because the concentration of the particles N is in the denominator of the equations, i.e. it would appear that as the concentration N decreases, the intensity of molecular scattering should increase which, evidently, contradicts the physics of the process. However, it should be mentioned that the polarisability α in the initial equation (5.1.11) is the characteristic of the molecule of matter, i.e. it is independent of concentration. However, the dependence on N is reflected in the refractive index of the gas because, according to (5.1.12), (n 2 –1) is proportional to N. Therefore, in complete correspondence with (5.1.11) and the physical meaning, the section of molecular scattering (5.1.13) is independent of the concentration of the particles and the volume coefficient of molecular scattering (5.1.14) is directly proportional to concentration.

Rayleigh law According to (5.1.14), the volume coefficient of molecular scattering is inversely proportional to the fourth power of the wavelength of light. This claim is referred to as the Rayleigh law. According to this law, blue and grey light rays (approximately 0.45 µm) are scattered in the air to a considerably greater extent (by almost a factor of 4) in comparison with orange and red rays (0.65 µm). This also explains the blue colour of the cloudless sky determined by the scattered solar radiation with the blue and grey rays being dominant. The extinction of the intensity of light as a result of molecular scattering is also stronger for the blue and grey rays in comparison 175

Theoretical Fundamentals of Atmospheric Optics

with the orange and red ones, as directly indicated by the Bouguer law (3.4.5). This also explains the red colour of the sunset and also the red colour of the setting Sun and the Moon: in the case of high zenith angle θ the power of the exponent increases and the extinction in the atmosphere becomes considerable and, consequently, in the spectrum of direct and scattered radiation, mainly red and orange rays are retained..

Volume coefficients and optical thicknesses of Rayleigh scattering The quantitative characterisation of Rayleigh scattering and extinction may be carried out using Table 5.1 which gives the volume coefficients of molecular scattering σ at pressure p = 1 atm and T = 15 ºC and the appropriate optical thicknesses (along the vertical) τ(0, ∞) of the entire atmosphere of the earth. Table 5.1 shows clearly the strong spectral dependence of the coefficient of Rayleigh scattering and optical thickness of the entire of atmosphere. If in the case of the wavelength of 0.30 µm the optical thickness of the atmosphere is higher than 1, in the near infrared range it does not exceed fractions of unity. This indicates the important role played by Rayleigh scattering in the extinction of, for example, solar radiation in the ultraviolet range of the spectrum and its low value in the infrared and even more so microwave spectral ranges. Usually, the molecular scattering in these longwave spectral ranges is ignored when solving different Table 5.1. Molecular scattering coefficients and optical thickness of the atmosphere along the vertical [24] Wavelength, µm

σ, km

τ(0,∞ )

–1

0.30

1.446×10

–1

1.2237

0.32

1.098×10 –1

0.9290

0.34

8.494×10

–2

0.36

6.680×10

–2

0.5653

0.38

5.327×10 –2

0.4508

0.40

4.303×10

–2

0.45

2.644×10 –2

0.50

1.726×10

–2

0.55

1.162×10

–2

0.0984

0.60

8.157×10 –3

0.0690

Wavelength, µm

τ(0, ∞ )

σ, km–1

0.65

5.893×10

–3

0.0499

0.70

4.364×10 –3

0.0369

0.80

2.545×10

–3

0.0215

0.90

1.583×10

–3

0.0134

1.06

8.458×10 –4

0.0072

0.3641

1.26

4.076×10

–4

0.0034

0.2238

1.67

1.327×10 –4

0.0011

2.17

4.586×10

–5

0.0004

3.50

6.830×10

–6

0.0001

4.00

4.002×10 –6

0.0000

0.7188

0.1452

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atmospheric optics problems. The strong effect of molecular scattering is found in the Venus atmosphere where the optical thickness of molecular scattering may reach hundreds of units.

Light polarisation in molecular scattering Having the scattering matrix (5.1.9), it is quite easy to calculate the polarisation of light during Rayleigh scattering. We convert the right upper square of the 2 × 2 matrix to the diagonal form using the equations (3.6.12)–(3.6.13): 1  2 2 0  2 (1 + cos γ − sin γ )   = 1   2 2 + γ − γ 0 (1 cos sin )     2 2  cos γ 0  = . 1  0

(5.1.15)

As explained in section 3 .6, the elements of the matrix (5.1.15) have the meaning of the coefficients of transformation of the components of the intensity of scattering radiation in the scattering plane I 0,|| and in the perpendicular direction I 0,⊥ . In many cases, the coefficients of the matrix (5.1.15) are referred to as the phase function of light scattering with linear polarisation in the scattering plane (x (γ) = cos 2 γ) and the linear polarisation in the plane normal to the scattering plane (x (γ) = 1). Attention should be given to the fact that in the latter case isotropic scattering takes place. These scattering phase functions are indicated by the dashed line in Fig. 5.2. Let us assume that the incident light with intensity I 0 is not polarised. Consequently, according to the transformation (3.6.13), both mutually perpendicular components for the light are equal to I 0,|| = I 0,⊥ = 1/2 I 0 . We should remember the definition of the degree of linear polarisation (3.5.21) as the ratio of the difference of the maximum and minimum intensities to their sum. After scattering the maximum intensity will be I ⊥ = 1⋅ I 0 , and the minimum intensity 1 2

1 I | = 1⋅ I 0 cos 2 γ (because in all cases cos 2 γ ≤ 1). Consequently, the 2

degree of linear polarisation of scattered light (3.5.21) is:

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Theoretical Fundamentals of Atmospheric Optics

Pl =

1 − cos2 γ sin 2 γ = . 1 + cos2 γ 1 + cos2 γ

(5.1.16)

Thus, the degree of linear polarisation in Rayleigh scattering is equal to zero at γ = 0 and γ = 180° and equals 100% at γ = 90°, i.e. in the directions normal to incident light, the scattered light is completely linearly polarised.

Polarisation of scattered radiation of the cloudless sky As discussed in detail in chapter 8, the scattering in the atmosphere of the Earth and planets is multiple and, consequently, equation (5.1.16) is valid only for the first scattering. Also, in addition to molecular scattering, there is also aerosol scattering. Therefore, the pattern of polarisation of the scattered light of the cloudless sky, obtained in measurements, differs from the ideal schema described previously. Firstly, the polarisation is actually maximum at points under the angle of 90° in relation to the Sun but the actual values of the degree of linear polarisation at these points are approximately 60–70%. Secondly, the zero value of polarisation is not obtained at the points 0 and 180° but at a ‘distance’ of approximately 15–20° from them. The upper point of zero polarisation (20° above the Sun) is referred to as the Babinet point, the lower point (20° below the Sun) as the Brewster point. At a low Sun, there is the third point of zero polarisation, i.e. the Arago point, which is 20° above the antisolar point (scattering angle 160°).

Clarification of molecular scattering theory In derivation of the equations of molecular scattering, the molecules were regarded as ideal spheres. However, because of the anisotropy in the structure of the molecules, the exact theory provides a correction for the resultant relationships in the form of a multiplier which depends on the depolarisation factor δ [20]. It appears that in molecular scattering, the degree of linear polarisation (5.1.16) for the angle of 90° is theoretically not equal to unity, and is lower by the value of δ in comparison with unity. The value of δ depends on the type of gas and for air δ = 0.035. The value of δ is used to express the correction multiplier in the equations of molecular scattering: for the volume coefficient:

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Light Scattering in the Atmosphere

σ=

8π3 (n2 − 1)2 6 + 3δ , 3N λ 4 6 − 7δ

(5.1.17)

and for the phase function

x( γ ) =

3 (1 + δ + (1 − δ)cos2 γ ). 4 + 2δ

6 + 3δ = 1.06 and, consequently, 6 − 7δ equation (5.1.17) is 6% more accurate than equation (5.1.14).

It should be mentioned that for air

Molecular scattering in the presence of absorption Previously, we examined molecular scattering in the pure form. However, it may be accompanied by radiation absorption. We examine again a molecule of air and an electromagnetic wave with the strength E 0 incident on on the molecule. It is assumed that the absorption of the molecule is determined by the transitions between the energy levels of only one electron. Remaining in the framework of classic electrodynamics, we examine this electron as an attenuating harmonic oscillator oscillating under the effect of the external field E 0 [62]. According to the second Newton law, the equation of motion of the electron may be presented in the form:

m

d2 x dx = −kx − q + eE0 , 2 dt dt

(5.1.18)

where m is the mass, e is the charge of the electron, –kx is the quasi-elastic return force, trying to restore the electron to the

dx is the force, identical to the friction force dt and introduced to take into account light absorption. Dividing by m, equation (5.1.18) is reduced to the form: equilibrium position; q

d2 x dx e + γ + ω20 x = E0 , 2 dt dt m

(5.1.19)

where γ = q/m is the extinction coefficient; ω02 = k/m is the natural cyclic frequency of oscillations of the electron ω 0 = π2ν 0 . It is now assumed that the field of incident radiation is again represented by a linearly polarised wave, and the spatial variation of the amplitude of the wave on the scales of the molecule is 179

Theoretical Fundamentals of Atmospheric Optics

ignored (as previously). The molecule is again regarded as a vibrating dipole carrying out forced oscillations with the frequency of the external field of ω 0 = π2ν, consequently, using the complex form of writing (3.5.4) we obtain x = x 0 exp(i2πνt).

(5.1.20)

substitution of (5.1.20) into (5.1.19) gives

−4π2 ν 2 x + 2πiνx + 4π2ν 02χ =

e E0 , m

and consequently

x=

e/m E0 . 4π2 (ν 02 − ν 2 ) + 2πiνγ

According to the definition of the dipole, moment P = ex, on the other hand, P = α∼ E 0 , and consequently we obtain for the polarisability of a single molecule:

α=

e2 / m . 4π2 (ν 02 − ν 2 ) + 2πiνγ

(5.1.21)

Further, we may use, as previously, the resultant value of α in all considerations and, consequently, the normalised matrix and the scattering phase function show no changes, and the equation for the scattering cross-section (5.1.11) changes to the form:

Cs =

128π5 αα* . 3λ 4

(5.1.22)

It should be mentioned that the dependence of the cross-section on the wavelength (5.1.22) is now complicated because α depends on ν = c/λ. Taking into account the relationship of the polarisability and the refractive index (5.1.12), and using the complex refraction index (CRI), introduced in chapter 3, n–iκ may be written in the following form:

(n − iκ)2 = 1 + 4πN α = 1 +

2 Ne 2 1 . 2 m 2π(ν 0 − ν 2 ) + ivγ

(5.1.23)

Separating from (5.1.23) the real n and imaginary κ parts of the CRI, we may easily obtain for them explicit expressions but they 180

Light Scattering in the Atmosphere

are very cumbersome. We shall use a simpler procedure: it is taken into account that CRI for these gases is very close to unity and, consequently, it is possible to write: (n –iκ) 2 –1 = (n – iκ – 1) (n – iκ + 1) ≈ 2 (n – iκ –1). Consequently, separating the real and imaginary parts of the fraction of CRI (5.1.23) we obtain

n − iκ = 1 +

Ne 2 2π(ν 02 − ν 2 ) − iνγ . m 4π2 (ν 02 − ν 2 )2 + ν 2 γ 2

(5.1.24)

The absorption in the model examined here is significant only in the vicinity of resonant frequency ν 0 . Consequently, we set ν ≈ ν 0 . Therefore, ν 20 –ν 2 = (ν 0 –ν) (ν 0 +ν) ~ 2ν 0 (ν 0 –ν). Consequently, we obtain the following equations for the real and imaginary parts of the CRI:

n = 1+

ν0 − ν 4πNe 2 , mv0 16π2 (ν 0 − ν) 2 + γ 2

Ne 2 γ κ= . mν 0 16π2 (ν 0 − ν) 2 + γ 2

(5.1.25)

Attention should be given to the equation (5.1.25) for the imaginary part of the CRI κ. Its dependence on frequency is identical to the Lorentz shape of the spectral line. The volume coefficient of absorption (denoted by β ν in this case) in the spectral line with the Lorentz shape is written in the form:

βν = kν N = NSf L (ν − ν 0 ) = NS

αL 1 . π (ν − ν 0 ) 2 + α 2L

On the other hand, in chapter 3 we determined the relationship between the volume coefficient of absorption and the imaginary part of the CRI (3.5.12):

κ = βν

c NS αL = 2c . 4πν 4π ν (ν − ν 0 ) 2 + α 2L

(5.1.26)

Comparing (5.1.26) with (5.1.25) and taking into account ν ≈ ν 0 , we formally obtain:

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Theoretical Fundamentals of Atmospheric Optics

γ = 4πα L ,

e 2 cS = . m π

(5.1.27)

Substituting (5.1.27) into (5.1.25), we obtain the expression for the CRI of the gas through the spectroscopic parameters:

n = 1+ N

Sc ν0 − ν , 2 4π ν 0 (ν 0 − ν) 2 + α 02

Sc αL κ=N 2 . 4π ν 0 (ν − ν 2 ) 2 + α 2L

(5.1.28)

We have derived relationships (5.1.28) within the framework of classical mechanics. However, in the condition of fulfilling the previously mentioned approximations and the equivalence of the shapes of the lines of absorption and radiation, discussed in chapter 4, quantum mechanics leads to the same expressions, only with the addition of summation in respect of all lines within the limits of some range close to ν 0 . Thus, the knowledge of the spectroscopic parameters of the gases makes it possible to calculate the volume coefficient of absorption and the volume coefficient of scattering.* Away from the absorption line ν 0 , the equations (5.1.28) are no longer applicable. However, in this case, we have the condition opposite to the condition used previously, |ν 20 – ν 2 | = |(ν 0 – ν)(ν 0 +ν)| >> ν, and in equation (5.1.23) we can ignore the term iνγ in the denominator and write immediately already for the real refractive index n:

n2 = 1 + N

e2 1 2 πm ν 0 − ν 2

or, taking into account n + 1 ≈ 2,

n = 1+ N

e2 1 , 2 2πm ν 0 − ν 2

(5.1.29)

where it is again necessary to carry out summation in respect of all the absorption lines. *The relationships indicate that the spectral dependences of the real and imaginary parts of the CRI are interconnected. In optics (by other way and with deriving some other final formulae – Kramer–Kronig relationships) it is proved that this claim holds for any substance [6].

182

Light Scattering in the Atmosphere

The refractive index of air The calculations of the CRI using equation (5.1.28) are relatively complicated and time-consuming. However, we take into account the fact that the molecular scattering, as reported previously, is significant only in the ultraviolet and visible ranges of the spectrum. Therefore, in the practical calculations we do not take into account the effect on the refractive index of air of molecular absorption in the infrared region ignoring consequently the error resulting from this procedure. In the ultraviolet and visible ranges we take into account absorption by air in the long-range ultraviolet range (Fig. 4.14) and different modifications of the equation (5.1.29). In particular, the approximate empirical relationship [20] is sufficiently popular

29498.1 255.4  + (n0 − 1) = 10 −6  64.328 + −2 146 − λ 41 − λ −2 

 , 

(5.1.30)

where λ is the wavelength in µm; n 0 is the refractive index at a pressure of p 0 = 1000 mbar, a temperature of T 0 = 15 ºC and zero humidity. Finally, taking into account (5.1.28) and (5.1.29), it is possible to use different relationships for the dependence of the refractive index of air on the concentration of molecules N. The simplest formula is [24]

n − 1 = (n0 − 1)

ρ , ρ0

(5.1.31)

where ρ is the density of air; ρ 0 is the density of dry air at p 0 and T 0 (ρ 0 = 1.20903 · 10 –3 g·cm –3 ).

5.2. Scattering and absorption on aerosol particles Aerosols optics To determine the characteristics of the interaction of aerosol particles with radiation, they are mathematically modelled by bodies of a specific geometrical form and, consequently, it is possible to solve for these bodies the problem of diffraction of electromagnetic waves on them. Thus, the optics of aerosols in the theoretical plan is closely linked with classic electrodynamics. 183

Theoretical Fundamentals of Atmospheric Optics

The main difficulty in the theoretical analysis of scattering on the aerosol particles is that, in a general case, the dimensions of the particles are no longer small in comparison with the wavelength of incident radiation (chapter 2 – the characteristic dimensions of the aerosols). Therefore, it is not possible, as in molecular scattering, to ignore the variations of the vector of the electrical strength of the incident wave on the particle surface. It is therefore necessary to calculate the characteristics of a heterogeneous electromagnetic field inside the particle which, taking into account the boundary conditions on the surface of the particle, is associated with the field of scattered radiation in which we are interested [6]. For an exact solution of this problem, it is necessary to solve the Maxwell equations which, even in the simplest cases, results in timeconsuming operations. After writing Maxwell equations and the boundary conditions, the solution of the equations is transformed into a purely mathematical problem. The solution itself, as will be shown, results in a very complicated dependence of the characteristics of scattering on the initial parameters and it is very difficult to understand the ‘physical meaning’ of the results. However, in aerosol optics, we can use approximations which make it often possible to obtain simple solutions of the problem of scattering [5]. For example, the Rayleigh–Hans–Jeans approximation is based on the assumption that the field inside the particle is homogeneous and is formed by dipoles with the same orientation; consequently, the external field can be determined by the superposition of the fields of all dipoles. This approximation is fulfilled quite satisfactorily for the particles with the dimensions considerably smaller than the wavelength. In the approximation of the ‘soft’ particles according to van de Hulst, it is assumed that the internal field of the particle coincides with the external field of the incident wave. This is fulfilled for the particles with the refractive index close to unity, in particular for the water particles. Examination using the approximations makes it possible to carry out physical analysis of the scattering processes.

Mie theory The simplest case for which the general solution of the diffraction problem has been obtained is light scattering by a homogeneous sphere. The solution is referred to as the Mie theory (according to German scientist Gustav Mie, who proposed this solution in 1908). We shall present the Mie equations in the form of final results, omitting the explanation because it is time consuming. 184

Light Scattering in the Atmosphere

Briefly, the method of the deriving the Mie equations may be described as follows [6]. We write the Maxwell equations for the incident wave, the scattered wave, and the wave passed through inside the particle, and the boundary conditions for them. Subsequently, using the well-known method of theoretical electrodynamics – the introduction of the scalar and vector potentials – the system of equations is transformed from the vector to scalar form. Because of the spherical symmetry of the particle, the solution is sought in the form of expansion into a series in respect of spherical functions, and the incident wave and boundary conditions are transformed to the same type. Consequently, the variables in the equations are separated, the equations are reduced to the cases with known solutions and for the coefficients of the series we obtain systems of easy solvable linear algebraic equations. The results are expressed through the Bessell functions with the half-integer index and Legendre polynomials. All the mathematical operations in the derivation of the Mie equations are quite simple but are accompanied by cumbersome transformations. This derivation is described in greater detail in [6]. The absorption and scattering of light by a homogeneous spherical particle is characterised by three dimensionless parameters: the ratio x = 2πr/λ, where r is the radius of the particle, and m is the complex refractive index of the material of the particle* (the CRI is the pair of the numbers and, consequently, there are three parameters). The equations for the characteristics of interaction are constructed on the basis of the coefficients of complex series a n and b n :

an =

mψ n (mx )ψ′n ( x ) − ψ n ( x )ψ′n (mx ) , mψ n (mx )ξ′n ( x ) − ξn ( x )ψ′n (mx )

ψ (mx)ψ′n ( x) − mψ n ( x)ψ′n ( mx) bn = n , ψ n (mx)ξ′n ( x) − mξn ( x)ψ′n (mx)

(5.2.1)

here ψ n (z) and ξ n (z) are the Riccati–Bessell functions in the general case from the complex argument; ψ' n (z) and ξ' n (z) are the *More accurately, if the wavelength of light is the wavelength in vacuum, then m is the ratio of the CRI of the particle to the real part of the refractive index of the medium. For particles in the atmosphere, the real part of the refractive index of the medium is always assumed to be equal to unity but, for example, for particles in water (hydrosols) one should use the ratio. 185

Theoretical Fundamentals of Atmospheric Optics

derivatives of these functions. The recurrent equations for calculating the Riccati–Bessell functions will be presented below. In aerosol optics, in addition to the extinction, scattering and absorption cross-sections, it is also necessary to introduce the factors of extinction, scattering and absorption Q e , Q s , Q a which are determined as the ratios of the cross-sections to the area of projection of the particle, perpendicular to the incident wave. For the sphere, this area is πr 2 and, consequently Q e = C e / πr 2 , Q s = Q s /πr 2 , Q a = C a /πr 2 . The factors are dimensional quantities and, consequently, make it possible to compare the relative characteristics of the interaction of particles of different dimensions (this possibility is the reason for introducting the factors). The Mie theory for the scattering Q s = Q s /πr 2 and extinction Q e = C e / πr 2 factors gives Qs =

2 x2

Qe =

∑ (2n + 1)(| a

2 x2

∞

n=1

n

|2 + | bn |2 ),

∑ (2n + 1) Re(a ∞

n=1

n

+ bn ).

(5.2.2)

The absorption factor is Q e – Q s . The matrix of scattering on a homogeneous spherical particle has the form (3.6.4). Its elements depend only on the scattering angle γ. The matrix is calculated using equation (3.6.3) for the complex coefficients S 1 and S 4 for which, as in molecular scattering, the vector E || is found in the scattering plane and the dependence of these coefficients on γ is associated with the ‘angular’ functions π n (γ) and π n (γ): S1 ( γ ) =

S4 ( γ) =

∑ n(n + 1 (a τ ( γ) + b π (γ )), ∞

n =1

2n + 1

n

n

n

n

∑ n(n + 1 (a π ( γ) + b τ (γ )). ∞

2n + 1

n

n =1

n

n

n

(5.2.3)

Since we have decided to use only the normalised scattering matrices, the constant multipliers in equation (5.2.3) are ignored. It is now only necessary to introduce the recurrent equations for the calculation of the Riccati–Bessell functions and angular functions π n (γ) and τ n (γ):

186

Light Scattering in the Atmosphere

ψ n+1 ( z ) =

2n + 1 ψ n ( z ) − ψ n−1 ( z ), ψ −1 = cos z , ψ 0 = sin z; z

ξn+1 ( z ) =

2n + 1 χ n ( z ) − χ n−1 ( z ), χ−1 = − sin z , χ0 = cos z; z ξ n ( z ) = ψ n ( z ) + iχ n ( z );

πn ( γ ) =

(5.2.4)

(2n − 1) cos γ n πn−1 ( γ ) − πn−2 ( γ ), π0 ( γ = 0), π1 ( γ ) = 1; n −1 n −1 τ n ( γ ) = n cos γπ n ( γ ) − ( n + 1)π n−1 ( γ ).

As already mentioned, the Mie equations (5.2.1)–(5.2.4) were obtained as a purely mathematical solution of the diffraction problem on a homogeneous sphere and do not make it possible to examine the physics of the process. Below, we present several results of calculations obtained using these equations but for the moment we continue theoretical examination of aerosol scattering and examine the limiting cases which would enable us to determine some important physical relationships.

Small particles Let us assume that the dimensions of the particles which, as previously, are regarded as homogeneous spheres, are considerably smaller than the radiation wavelength. In this case, we can, as in the case of molecular scattering, ignore the inhomogeneity of the external field incident on the particle. In addition to this, it is assumed that the matter of the particle is a dielectric material, i.e. the conductivity of matter is either non-existent or insignificantly small. Because of the phenomenon of polarisability of the dielectric (the matter of the particle), the charges induced by the external field start to appear on the surface of the particle. Because of the spherical symmetry of the particle, the homogeneity of the external field and the absence of conductivity, the positive and negative charges gather in different hemispheres of the particle and are distributed strictly symmetrically in relation to each other. The distribution of the charges means that we have an emitting dipole [6]. Further, it is necessary to repeat all the considerations, used in the derivation of the equations of molecular scattering and,

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Theoretical Fundamentals of Atmospheric Optics

consequently, all the relationships (5.1.1)–(5.1.11) are valid for the small aerosol particles. To determine the cross-section of scattering by a small particle, we use the expression available in electrostatics for polarisability of a homogeneous sphere in a homogeneous field:

α = r3

ε − 1 3 n2 − 1 , =r 2 n +2 ε+2

(5.2.5)

where r is the radius of the sphere; n is the refractive index of the material of the sphere. Substituting (5.2.5) into (5.1.11) we obtain

128π5 r 6  n 2 − 1  Cs = . 3 λ 4  n 2 + 2  2

(5.2.6)

The above equation gives the same inverse proportionality of the scattering cross-section of the fourth degree of the length of the wave as molecular scattering. This means that the Rayleigh law is valid for the small aerosol particles. In addition to this, previously we have mentioned that the phase function and the scattering matrix of the small particles and, consequently, their polarisation characteristics, are also identical to the molecular characteristics. Consequently, we are talking about Rayleigh aerosol particles (for which the above approximation is fulfilled), the Rayleigh phase 3 function of scattering (the phase function x(γ ) = (1 + cos2 γ ) , Fig. 4 5.2), the region of Rayleigh scattering (the range of wavelengths and size of the particles in which the Rayleigh approximation is fulfilled). Since the boundaries of approximation of the Rayleigh scattering depend on the wavelength of light, in the long-range infrared and microwave ranges Rayleigh scattering also takes place in the case of verylarge particles of clouds and precipitation. It should be mentioned that the coincidence between the relationships of scattering for the molecules and small particles is not accidental. Its physical basis is the approximation of the homogeneity of the field, incident on the particle. This approximation is referred to in the literature as dipole approximation, or electrostatic approximation, Rayleigh approximation, or RayleighHans–Jeans approximation. Since the fluctuations of the density of air in the normal conditions are also small in comparison with the wavelength of light, this approximation is also fulfilled for them. This is the physical reason for the coincidence of the formulae of 188

Light Scattering in the Atmosphere

molecular and fluctuation theories of scattering by gases.

Small particles as a limiting case of Mie theory The previously made conclusions on the scattering by small particles should evidently be confirmed by a general theory, Mie theory. It

2πmr > n (absorption band) R tends to unity. For example, if the surface is illuminated by radiation in a relatively wide spectral range, the reflection spectrum will contain maxima corresponding to the position of the absorption bands of substances included in the composition of the underlying surface. In the literature, this effect is referred to as the re-emission effect and can be used to determine the physical–chemical nature of the underlying surface on the basis of the reflection spectra. There is a special angle θ B – Brewster angle, at which R v = 0. Only the radiation for which the strength of the electrical field is normal to the plane of incidence is reflected at this angle. Therefore, in the incidence of non-polarised solar radiation on the flat surface under the Brewster angle, the reflected light is completely polarised in the direction parallel to this plane. The Brewster angle is determined by equation θ B = n. Figure 6.4 shows an example of the angular dependence of the reflection coefficients of a smooth sea surface as a function of the angle of incidence for two wavelengths – in the visible range (λ = 0.55µm) and in the microwave range (λ = 3 cm). In the visible range, the refractive index is close to 1.2–1.3 and the reflection coefficients at nadir (vertical) incidence are very small, 0.01–0.02 up to the angles of incidence of 40–60º. At large angles of incidence (θ > 80º) the reflection coefficients rapidly increase and when the angle of incidence approaches 90º, the reflection coefficients tends to unity. Figure 6.4. shows for vertical (parallel) polarisation the Brewster angle ~53º at which reflection for this component is equal to zero. In the microwave range at λ = 3 cm the complex refractive index is approximately equal to m = 52 – 37i and the reflection coefficient for vertical incidence is high and equals 0.61. The Brewster angle is approximately 82º.

257

Emission coefficient

Reflection coefficient

Theoretical Fundamentals of Atmospheric Optics

µm

Angle of incidence, deg Fig.6.4. Example of the dependence of the reflection coefficients of the smooth sea surface on the angle of incidence for visible (λ = 0.55 µm) and microwave (λ = 3cm) ranges [116].

6.3. Quantitative characteristics of reflection of radiation (real surfaces) There are various methods for describing the optical characteristics of real underlying surfaces in the theory of transfer and in atmospheric optics. As in the case of the flat surface of the interface between two media (Fresnel equations), we start with the case of illumination of the surface by radiation in one direction (with the intensity I 0 or the radiation flux F 0 with the same numerical value). The angles, determining the direction of incidence radiation, are denoted by θ 0 – the zenith angle of incidence, and ϕ 0 – the azimuth of incidence; the angles which determine the direction of reflection will be denoted by θ and ϕ. As mentioned previously, in a general case, reflection from the surface is of the diffusion type. The more general characteristic of reflection is the two-directional coefficient of reflection r which depends both on the angles of incidence (θ 0 , ϕ 0 ), and on the direction of reflection (θ, ϕ). It is determined using the equation I(θ, ϕ) = r(θ, ϕ, θ 0 , ϕ 0 )I 0 , 258

(6.3.1)

Optical Properties of Underlying Surfaces

where I(θ,ϕ) is the intensity of radiation reflected by the surface in the direction (θ,ϕ). Because of the arbitrary nature of the selection of method of determination of the azimuths, the physical quantity (two-directional coefficient of reflection) cannot depend on its absolute value. Therefore, the two-directional coefficient of reflection is a function of three variables: r(θ, ϕ, θ 0 , ϕ 0) = r(θ, θ 0 , ϕ – ϕ 0 ).It is usually assumed that ϕ 0 = 0, i.e. all azimuths are counted from the azimuth of incidence. Therefore, finally I(θ, ϕ) = r(θ, θ 0 , ϕ)I 0 .

(6.3.2)

The previously examined partial case – mirror reflection – is written in the form r (θ, ϕ , θ0 , ϕ 0 ) =

{

R ( θ0 ) at θ=θ0 ,ϕ =ϕ 0 , 0 at θ≠θ0 ,

(6.3.3)

where R(θ 0 ) is the Fresnel coefficient of reflection (equation (6.2.10)). Equation (6.3.3) is often written in the form r(θ, ϕ, θ 0 , ϕ 0 ) = r(θ 0 )δ (θ 0 – θ)δ (ϕ – ϕ 0 ), (6.3.4) where δ(ϕ – ϕ 0 ) is Dirac’s function. It is often convenient to examine the radiation flux falling on the horizontal area, F = F 0 cosθ 0 instead of the intensity of radiation or flux directed to the area normal to the direction of radiation. The characteristics corresponding to the flux is referred to as the coefficient of (spectral) brightness (CSB) of the surface and is determined by the equation [47, 64, 65]

1 I (θ, θ0 , ϕ) = ρ(θ, θ0 , ϕ) F0 cos θ0 . π

(6.3.5)

It is needed to determine the relationship between the previously introduced characteristics of reflection:

r (θ, θ0 , ϕ) = ρ(θ, θ0 , ϕ )

cos θ0 . π

(6.3.6)

In the theory of radiation transfer the characteristics of reflection (in this case not only by the surface but also by the entire atmosphere–surface system) are described by the reflection function y(θ,θ 0 ,ϕ) [65], determined by the equation

I (θ, ϕ )cos θ =

1 y(θ, θ0 , ϕ )I 0 cos θ0 . 2π

259

(6.3.7)

Theoretical Fundamentals of Atmospheric Optics

Using definitions of different characteristics of reflection we have

ρ(θ, θ0 , ϕ ) =

1 y(θ, θ0 , ϕ ), 2 cos θ

cos θ0 1 y(θ, θ0 , ϕ ) . 2π cos θ

r (θ, θ0 , ϕ ) =

(6.3.8)

(6.3.9)

In many problems of the theory of radiation transfer and atmospheric optics it is interesting to examine the total energy of reflected radiation, i.e. the upwelling radiaion flux on the surface, instead of detailed (angular) characteristics of reflection

F↑ =

∫

2π

dϕ

∫ I (θ, ϕ)cos θ sin θ dθ.

π /2

0

(6.3.10)

0

The ratio of the upwelling and downwelling (falling) radiation fluxes is referred to as the albedo of the surface: A=

F↑ F↓

,

(6.3.11)

where F ↓ and F ↑ are the fluxes of the incident and reflected radiation. The albedo of the surface, often expressed in percent, gives the fraction of the incidence energy reflected by the surface. Correspondingly, the quantity (1–A) is the fraction of absorbed (and in a general case of transparent media – transmitted) energy. Case A=1 corresponds to the absolutely ‘white’ surface, A = 0 to the absolutely ‘black’ surface. On the basis of the definition of the albedo, we obtain the following relationships between the introduced reflection characteristics:

1 A= π

∫ dϕ ∫ ρ(θ, θ ,ϕ)cos θ sin θ dθ,

2π

π/2

0

0

0

1 A= cos θ

∫ dϕ ∫ r (θ, θ , ϕ)cos θ sin θ dθ,

2π

π /2

0

0

0

260

(6.3.12)

(6.3.13)

Optical Properties of Underlying Surfaces

1 A= 2π

∫ dϕ ∫

2π

π/2

0

0

y(θ, θ0 , ϕ )sin θ dθ.

(6.3.14)

We examine a case of orthotropic reflection for which according to definition I(θ,ϕ)=const. According to equation (6.3.5), this denotes the absence of the angular dependence of CSB on angles θ and ϕ. Consequently, we obtain r(θ,θ 0 ,ϕ)=A, i.e. for the orthotropic surface, the coefficient of spectral brightness of the surface is numerically equal to its albedo. The relationship (6.3.14) gives the physical meaning of the reflection functions y(θ,θ 0 ,ϕ). Since the integral from y(θ,θ 0 ,ϕ) over all directions is the albedo of the surface which because of equation (6.3.11) may be interpreted as the probability of reflection, the reflection function y(θ,θ 0 ,ϕ) is the density of the probability of reflection in the direction (θ,ϕ). In this sense, function y(θ,θ 0 ,ϕ) is some analogue of the scattering phase function (see Chapter 2).

1 y(θ,θ 0 ,ϕ) is A sometimes referred to as the reflection phase function. An important property of the coefficient of spectral brightness is its symmetry (the rule of reversibility or reciprocity, Helmholtz rule):

Therefore, this function or its normalized value

ρ = (θ,θ 0 ,ϕ) = ρ(θ 0 ,θ,ϕ).

(6.3.15)

This relationship may be proved by simple considerations. The entire real reflecting surface is broken up into a set of elementary areas and mirror reflection takes place from each area. Therefore, the direction of reflection (θ,ϕ) is determined by the orientation of the appropriate elementary area. In the coordinate system in which the elementary area is horizontal, the Fresnel equation and the law according to which the angle of reflection is equal to the angle of incidence) show evidently the equality of the reflected intensities when the direction of reflection is replaced by the direction of incidence. This equality should also be returned in the initial coordinates. However, here it must be taken into account that the intensity of radiation is determined as the energy per unit surface area. Therefore, the equality of intensity should be understood taking into account the reduction of these intensities to the unit area of the reflecting surface which, as indicated by the elementary geometry leads to the equation

261

Theoretical Fundamentals of Atmospheric Optics

I (θ, θ0 , ϕ ) I (θ0 , θ, ϕ ) = cos θ0 cos θ

(6.3.16)

From this relationship and taking into account equation (6.3.5) we directly obtain the rule of reciprocity (6.3.15). Taking into account the equation (6.3.15), for other detailed characteristics of reflection we have the following relationship r(θ,θ 0 ,ϕ)cosθ = r(θ 0 ,θ,ϕ)cosθ 0 ,

(6.3.17)

y(θ,θ 0 ,ϕ)cosθ 0 = y(θ 0 ,θ,ϕ)cosθ,

(6.3.18)

Another characteristic of reflection from the underlying surfaces is the parameter of anisotropy of reflection determined by the following equation:

γ (θ, ϕ, θ0 , ϕ0 ) =

π r (θ, ϕ, θ0 , ϕ0 ). A

(6.3.19)

It is assumed that we are concerned here with a non-Lambert surface with albedo A. Consequently, the parameter γ(θ,ϕ,θ 0 ,ϕ 0 ,) gives the ratio of the intensity of the radiation, reflected by the surface, to the intensity of the radiation reflected from the Lambert surface with the same albedo A. This parameter may be higher than unity for the surface reflecting in specific directions more than the Lambert surfaces, and vice versa. It is very important to mention that all the previously examined optical characteristics of reflection are functions of the wavelength, as already demonstrated on the example of the Fresnel reflection coefficients. Therefore, when discussing the optical characteristics of some surface, it is necessary to specify the spectral range or spectral region to which this characteristic belongs. Previously, it was assumed that directional radiation is incident on the surface. We examine a more general and more appropriate case in which the surface is illuminated by diffusion radiation consisting of the direct and scattered (by the sky) solar radiation. For this case, we also introduce the concept of the coefficient of spectral brightness and albedo but in this case the incidence flux is the total flux consisting of the direct and scattered solar radiation. Sometimes these different definitions of the CSB and albedo are used without taking these differences into account. If the incident flux is the total radiation, then CSB and the albedo depend not only on the properties of the surface but also on the properties of the 262

Optical Properties of Underlying Surfaces

atmosphere. For example, if we write in detail a radiation flux incident on the surface (according to the equation (6.3.10) for an upwelling flux), and the definitions of the CSB and the albedo give directly their dependence on the angular distribution of the downwelling radiation I ↓ (θ 0 ,ϕ 0 ) incident from the atmosphere onto the surface. Naturally, according to these considerations, the albedo, determined for the total incident radiation flux, is the function of the zenith angle of the Sun. Therefore, although this is not always the case, all measurements of the albedo should be accompanied by at least a qualitative description of the state of the atmosphere and by the definition of the zenith angle of the Sun. Thus, in examination of the surfaces of planets the surface albedo is not an unique characteristic of the optical properties of the underlying surface and it is a function of the atmospheric state and the conditions of illuminations of the underlying surface–atmosphere system. It should be mentioned that in actual experiments aimed at examining the optical properties of the underlying surfaces measurements are taken of just these quantities (related to the total flux of incident radiation). The albedo, determined by this definition, does not depend greatly on the condition of the atmosphere (the presence of clouds, aerosol) whereas the CSB may greatly differ for different conditions. In the theory of radiation tansfer we examine the characteristics of reflection of the entire atmosphere–underlying surface system [47, 65]. These characteristics can be regarded as functions of the altitude in the atmosphere, for example

A = (z) =

F ↑ ( z) F ↓ ( z)

ρ(θ, θ0 , ϕ, z ) = π

,

I (θ, ϕ ) F ↓ ( z)

(6.3.20)

.

(6.3.21)

_ According to this definition, the albedo A (z = 0)≠A, i.e. the albedo of the atmosphere–underlying surface is not the albedo of the surface, although this is often disregarded. The albedo and the CSB at the upper boundary of the atmosphere are very important in atmospheric optics. According to the definition

263

Theoretical Fundamentals of Atmospheric Optics

I (θ, ϕ, z = ∞) =

1 ρ(θ, θ0 , ϕ, z = ∞) F0 cos θ0 , π

A( z = ∞) =

F ↑ ( z = ∞) , F0 cos θ0

(6.3.22)

(6.3.23)

where F 0 cosθ 0 is the extra-atmospheric flux _ of solar radiation incident on a horizontal area. The quantity A (z= ∞) is the fraction of the energy reflected by a planet into space and is referred to as a flat albedo [47,65]:

1 A( z = ∞) = π

∫ dϕ ∫ ρ(θ, θ , ϕ, z = ∞)cos θ sin θ dθ.

2π

π/2

0

0

0

(6.3.24)

The flat albedo depends on the examined point of the planet and the angles of incidence of solar radiation. For the characteristic of reflection of solar radiation from the planet as a whole we should use the concept of the spherical albedo A s [47,65]. If a planet is regarded as a sphere with radius R 0 , the angle of incidence of the solar rays θ 0 for the sphere changes from 0 to 90º. Therefore, the ratio of the energy of the solar radiation reflected from the entire planet (from the area of the illuminated part 2πR 0 2 ) to the entire energy of solar radiation incident on the planet (on the projection area πR 02 ), i.e. spherical albedo, is determined by the relationships

As =

1 π

∫

2π

0

dϕ

∫ ∫ ρ(θ, θ , ϕ, z = ∞) ×

π/2 π/2

0

0

0

× cos θ cos θ0 sin θ sin θ0 dθdθ0 .

(6.3.25)

In conclusion, we present Table 6.1 which characterises different methods of description of the optical characteristics of the surface of the atmosphere–underlying surface system.

6.4. Examples of the optical characteristics of underlying surfaces We presented previously the Fresnel coefficients of reflection for the smooth surfaces, in particular, for the water surface. We analysed in greater detail the optical characteristics of different surfaces, paying attention to their complicated dependencies on the physical–chemical properties of the surfaces, the altitude of the Sun 264

Optical Properties of Underlying Surfaces Table 6.1. Methods of describing the optical characteristics of the surface or of the atmosphere-underlying surface system Inc id e nt ra d ia tio n

Re fle c te d ra d ia tio n

C ha ra c te ristic

Inte nsity (flux o f d ire c tio na l ra d ia tio n)

Inte nsity

Two - d ire c tio na l re fle c tio n fa c to r; c o e ffic ie nt o f (sp e c tra l) b rightne ss, re fle c tio n func tio n

F lux (to ta l)

Inte nsity

C o e ffic ie nt o f (sp e c tra l) b rightne ss

Inte nsity (flux o f d ire c tio na l so la r ra d ia tio n)

F lux re fle c te d a t a sp e c ific p o int o f p la ne t

F la t a lb e d o

Inte nsity (flux o f d ire c tio na l so la r ra d ia tio n)

F lux re fle c te d fro m e ntire p la ne t

S p he ric a l a lb e d o

F lux (to ta l)

F lux

Alb e d o

and the atmospheric state. The data for the actual surfaces are determined in theoretical and numerical modelling of these surfaces and also by measurements in field and laboratory conditions.

Theoretical models We describe briefly a scheme of determination of the theoretical reflection characteristics for the case of a wind-driven water surface. This surface is simulated by different methods. Two approaches to describing the complex spatial–temporal pattern of waves on the water surface. The first of them consists of the representation of the relief of the sea surface by regular functions of the spatial–temporal variables [85]. Thus, assuming that the amplitude of the wave is infinitely small in comparison with its length, the profile of the wave may be approximated by a sinusoidal. The second approach, statistical, is based on describing the characteristics of the surface of the sea in the form of the probability laws of distribution of different elements of the waves– altitude, period, length, slope of inclination, etc. For example, the model proposed by Cox and Munk [85] gives the probability of distribution of the slopes of the surfaces of the waves in the form of a Gaussian distribution (assuming that the distribution is independent of the direction of wind – isotropic case): 265

Theoretical Fundamentals of Atmospheric Optics

 ( z x2 + z 2y )  1 P(z x , zy ) = exp  − , πσ 2 σ 2  

(6.4.1)

where z x and z y are the characteristics of the slopes in respect of two orthogonal directions; σ is the dispersion of the slopes (independent of the direction) and is determined by the velocity of the wind v: σ 2 = 0.003+0.00512v ± 0.004.

(6.4.2)

The approximations for P(z x , z y ) and σ 2 were determined from the experimental data and held in the range of the wind speed of 0–14 m/s. This was followed by examination of the reflection (or radiation) from an arbitrary oriented area of the wind-driven surface of water. The characteristics of reflection of the wind-driven surface of the sea are determined by averaging taking into account the probability of distribution of the slopes of the water surface. In the presence of foam on the water surface, formed at high wind speeds, the model is more complicated and the medium near the surface is regarded as heterogeneous consisting of different components – water and air. In examination of the optical characteristics of the real land surfaces, it is necessary to construct different models of the surface layer of land. These models include the characteristics of the spectral distribution of the heterogeneities and dielectric permittivity of different components. Various models of the transfer of radiation in these media are then used. In the simplest case for infrared and microwave ranges the emissivity of the surfaces is determined by solving the integral equation of transfer of radiation without taking scattering into account. In more generalized models, the effects of scattering in the investigated inhomogeneous media are taken into account.

Classification of the spectra of reflection of natural surfaces A large number of experimental data is available for different optical characteristics of different underlying surfaces. There are detailed data on the albedo and CSB of different surfaces because these measurements are relatively simple. As an example, Fig.6.5. gives the values of the albedo of different surfaces in relation to the altitude of the Sun. Figure 6.5 shows that the variations of the albedo are most significant at low altitudes of the sun. The figure

266

Optical Properties of Underlying Surfaces

Snow (fresh)

Albedo

White sand Variability for snow Desert sand Dry grass (semi desert) Forest, Eucalyptus Still water Sun altitude, deg Fig.6.5. Dependence of the albedo of different surfaces on the altitudes of the Sun [111].

also shows the strong variability of the albedo of snow at high altitudes of the Sun. The spectral special features of the albedo and CSB differ quite considerably. However, it appears that regardless of the differences in the spectral albedo of the underlying surfaces, visible and near infrared ranges of the spectrum, those can be grouped into four main classes. This classification was proposed by E.L. Krinov on the basis of the results of the first aircraft measurements of the albedo carried out in the former USSR in the forties of the previous century [36]. The characteristic spectra of the albedo of the four classes are given in Fig. 6.6. The first class includes snow and clouds. Their albedo is high and slightly increases along the spectrum from ultraviolet to the beginning of the near infrared range of the spectrum. The second class includes soil, sand and open rock. They are characterised by a smooth, almost linear increase of the albedo with increasing wavelength. The third class is the water surface. The albedo of the water is small and its spectrum is almost constant or slightly decreases with increasing wavelength. Finally, the fourth class is plants. The albedo of green plants is characterised by a complicated spectral dependence: a local maximum in the range 0.55 µm causing its green colour, followed by a decrease and rapid increase after 0.7 µm, where the albedo of the plants is close to that of snow and clouds. Yellow plants (dry 267

Theoretical Fundamentals of Atmospheric Optics

λ µm Fig.6.6. Classification proposed by E.L. Krinov for the spectral dependences of the reflecting characteristics of natural surfaces [36]. 1) Snow and cloud; 2) soil, sand, rock; 3) water surfaces; 4) vegetation (characteristic spectral albedo representatives of these classes are given).

grass in the steppes, autumn leafy forests) have a similar spectal dependence but the maximum of the albedo in the vicinity of 0.55 µm is less distinctive. To provide more detailed characteristics of the albedo and its spectral dependences, we present Table 6.2 and 6.7.

Variability of albedo As already mentioned, the albedo of the surfaces is dependent on their type and state, the altitude of the sun and also the atmospheric state (the presence of clouds). We examine the variability of the albedo in relation to different factors, with special reference of the albedo for the solar range of the spectrum. Almost all surfaces are characterised by one special feature – the largest changes of the albedo take place from sunrise to altitudes of the sun of 20–30º. Sharp changes of the albedo of the underlying surface are detected only in the periods of thawing and the formation of snow, i.e. at changes of the type of underlying surface. In these periods, the 268

Optical Properties of Underlying Surfaces Table 6.2. Values of the albedo of surfaces (in percent) for the visible ranges of the spectrum. S urfa c e typ e s

C o nd itio ns

Wa te r surfa c e s

Eq ua to r Winte r, 3 0 o la titud e Winte r, 6 0 o la titud e S umme r, 3 0 o la titud e S umme r, 6 0 o la titud e

S no w

F re s h O ld

Alb e d o 6 9 21 6 7 75–95 40–70

S e a ic e

–

30–40

S a nd (d une s)

Dr y Mo ist

35–45 20–30

S o il

Da r k Gre y, mo ist Dry, gre y c la y Dry light sa nd

5–15 10–20 25–35 25–45

C o nc re te

Dr y

17–27

Ro a d

Bla c k

5–10

De s e r t S a va nna h

– Dr y s e a s o n Humid se a so n

Bush

–

25–30 25–30 15–20 15–20

Me a d o w

Gr e e n

10–20

F o re s t

Le a fy C o nife ro us

10–20 5–15

Tund ra

–

15–20

Gra in c ulture s

–

15–25

difference between the values of the albedo in adjacent days may reach 20–30º. The albedo of a dry snow cover with the cloudless sky varies in the range 52–99%. The albedo of a dirty, wet snow may decrease to 20–30%. With increase of the amount of clouds, the albedo of the snow surface increases. In the presence of a continuous cloud cover it may increase by 2–10%. The albedo of melting snow decreases to 30–40%. The albedo of the grass cover varies from 12 to 28% depending 269

Albedo

Theoretical Fundamentals of Atmospheric Optics

Wavelength, nm Fig.6.7. The spectral dependence of the albedo of different surfaces [111]. 1) Snow, altitude of Sun 38º; 2) wet snow, altitude of sun 27º; 3) lake water, altitude of sun 56º; 4) soil after melting of snow, altitude of sun 24º 30’; 5) wheat after ensilage, altitude of sun 54º; 6) tall green wheat, altitude of sun 56º; 7) yellow wheat, altitude of sun 46º; 8) Sudanese grass, altitude of sun 52º; 9) black soil, altitude of sun 40º; 10) stubble, altitude of sun 35º.

on the density, colour and moisture of the grass. The albedo of wet grass is 2–3% lower than that of dry grass. The reflectivity of the grass cover also depends on the altitude of the Sun and, consequently, the albedo of dry green grass in morning and evening hours is 2–9% higher than at midday. In the spring, the albedo of dry grass cover and last year’s grass varies in the range 10–24%. The reflecting properties of the surfaces, free from the vegetation cover, depend on the type of soil, structure and moisture content. The albedo of non-moistened soil is 8–26%. The highest albedo is shown by white sand, up to 40%. The albedo of wet soil is 3–8% lower than that of the albedo of the dry soil, that of the white sand 270

Optical Properties of Underlying Surfaces

by 18–20%. With a decrease of the surface roughness soil, the albedo increases. During the day the albedo of the soil changes from the maximum values at low altitudes of the Sun to minimum values at midday. The amplitude of the daily variations of the albedo of soil is 11–17%. The albedo of the surface of water depends on a number of factors: the altitude of the sun above the horizon, the ratio between the direct and diffusion components of solar radiation, the amount of clouds, the strength of waves and the characteristics of water reservoirs – depth, transparency of water, etc. The daily variation of the albedo of the water surface is most distinctive in the absence of waves when its amplitude may reach 30% or more. In the presence of strong waves, the albedo remains almost constant throughout the day. Under a continuous cloud cover, the daily variation of the albedo is also minimum. Waves and cloud decrease the albedo of the water surface at altitudes of the sun smaller than 30º. At a large altitude of the Sun, the clouds and waves have the reversed effect which is however considerably weaker. We present a number of specific examples of the spectral reflecting properties of different underlying surfaces [115, 116]. Figure 6.8a shows the spectral behaviour of the albedo of two agricultural cultures – wheat prior to the heading period and soya beans in comparison with the spectral characteristics of two types of bare soil – dry and wet. The figure shows clearly the variability of the optical characteristics of soils (wet soil has lower albedo values) and different types of vegetation. Figure 6.8b gives the spectral characteristics of reflection of a lucerne field in different periods of the life cycle. The curves shown in the graph correspond to different amounts of the biomass and different degrees of cover of the lucene soil – the case of exposed soil corresponds to the zero values of the biomass and the covering. The transformation of the spectral curves of reflection with increase of the biomass and the covering makes it possible to measure the reflected solar radiation for inspection of the state of cultures. It is important to note a very important special feature of the spectral behaviour of reflection from the vegetation cover, shown clearly in Fig. 6.8a and Fig. 6.8b – a large increase of the albedo in the vicinity of 0.8 µm. The green colour of vegetation is connected with this special feature. The presence of chlorophyll in the plants results in strong absorption of solar radiation for wavelengths shorter than 0.7 µm. The variability of the spectral characteristics of the reflection of leaves of a plane tree in relation to their moisture content is shown 271

Theoretical Fundamentals of Atmospheric Optics a

Wheat prior to heading, 80% cover

Albedo

Soya beans – 90% cover Dry, bare soil Moist, bare soil

Wavelength, µm Cover, %

Albedo, %

b

Green biomass, kg/ha

Wavelength, µm Fig.6.8. Spectral behaviour of albedo [114] of two agricultural cultures – wheat prior to the heading period, and soya beans in comparison with spectral characteristics of two types of bare soil – dry and moist (a) and spectral characteristics of reflection [115] of a lucerne field in different periods of the life cycle (b).

in Fig. 6.9. With increase of the moisture content the albedo of the leaves rapidly decreases in the near infrared range of the spectrum. It should be mentioned that the albedo values in this figure are reduced to the albedo of a specially prepared sheet of magnesium oxide. 272

Reflection coefficient (%) (in relation to MgO)

Optical Properties of Underlying Surfaces

Moisture content, %

Wavelength, µm Fig.6.9. Variability of spectral characteristics of reflection of plane tree leaves in relation to the moisture content [115] (in relation to the reflection coefficient of MgO).

The two-directional reflection coefficients are characterized in Fig. 6.10 which gives the factors of the anisotropy of the reflection of the atmosphere – underlying surface system obtained as a result of satellite measurements. (As mentioned previously, the concept of the albedo and other reflection characteristics can be applied directly to both the underlying surfaces and to the entire atmosphere – surface system). The figure shows two cases of underlying surface – the ocean (Fig. 6.8a) and the snow cover (6.8b). The factors of anisotropy of reflection are given as functions of the observation zenith angle of satellite devices (in relation to the local vertical) for eight sub-intervals of the azimuthal angle in relation to the azimuths of the sun. These measurements relate to cases in which the zenith angle of the sun is in the range 25.84–36.87º. The maximum values of the factor of anisotropy of reflection above the ocean with a cloudless sky were recorded for the angles of examination in the vicinity of 30º and the range of azimuthal angles of 0–30º and correspond to the examination of solar track (quasi-mirror reflection). Another special feature of the reflection is the increase of the factor of anisotropy of reflection with increase of the angle of examination (when approaching approach to the horizon of the planet) when the azimuth of observation is in the angle range 90–180º, i.e. on approaching the 273

Theoretical Fundamentals of Atmospheric Optics

Anisotropy factor

a

Azimuth range

Azimuth range

Anisotropy factor

b

Azimuth range

Azimuth range

Zenith angle of observation, deg Fig.6.10. Factors of reflection anisotropy for the atmosphere–underlying surface system as a function of zenith angles of the Sun, measured in satellite measurement above the water surface (a) and snow (b) [114]. Observation azimuth ranges, deg: 1) 0–9°; 2) 9– 30°; 3) 30–60°; 4) 60–90°; 5) 30–120°; 6) 120–150°; 7) 150–171°; 8) 171–180°.

opposite direction from the sun. Figure 6.10b corresponds to the indentical examination for the snow surface. In this case, the reflection characteristics are more homogeneous in comparison with the water surface.

6.5. Emitting properties of underlying surfaces In the infrared and microwave regions of spectrum, the underlying surfaces of the planets are important sources of the generation of natural radiation. The characteristics of these surfaces as emitters are described by the coefficients of radiation or the emissivity of the surfaces [34, 45, 85, 102, 1156, 119]. The emissivity of a surface (λ,T) is the ratio of intensity of radiation of the surface with temperature T to the radiation of the absolutely black body at the same temperature 274

Optical Properties of Underlying Surfaces

ε (λ , T ) =

I s (λ, T ) . B(λ, T )

(6.5.1)

The radiation of a black body is isotropic which cannot be said about the radiation of real surface. Therefore, in a general case, the emissivity depends on the direction of radiation, wavelength and in a number of cases the temperature. Of course, the emissivity also depends greatly on the physical–chemical properties of the surface – its nature, form of the surface, etc. An important relationship used widely in atmospheric optics is a relationship between the absorption, reflection and emissivity of the medium. To derive this relationship, the conversion of the radiation energy during its interaction with the medium is examined. The incident radiation may be reflected from the medium, be absorbed by it, and also part of the radiation may transfer to the medium. To characterise the processes, the reflectivity (the albedo of the medium) A λ, the absorptivity B λ, the transmission function P λ are introduced. They are the ratios of the appropriate components of radiation to incident radiation. According to the law of conservation of energy, the sum of these quantities should be equal to unity A λ + B λ + P λ = 1.

(6.5.2)

It is implicitly assumed that in these processes we can ignore phenomena such as Raman scattering and fluorescence (see Chapter 5) which lead to the redistribution of radiation energy over wavelengths. Assuming that the entire radiation incident on the medium is absorbed (P λ=0), we obtain a simpler relationship A λ + B λ = 1.

(6.5.3)

Further, in examining the relationship between different characteristics of the medium we use the Kirchoff law defining the relationship between the emissivity and absorbing properties of the medium. Here it is confirmed that B λ = ε λ. Consequently, equation (6.4.3) gives the relationship between the emission and reflecting characteristics of medium ε λ =1–A λ .

(6.5.4)

It must be remembered that this ratio holds for a fixed wavelength. The emissivity of the underlying surfaces strongly depends on their type, the form of the surface, the wavelength and the angle of observation. To determine ε λ we use the experimental and numerical methods. In the latter case, equation (6.5.4) is used 275

Theoretical Fundamentals of Atmospheric Optics

widely. For example, the Fresnel coefficients of reflection for the smooth surface (examined in paragraph 6.2) enable us to determine its emissivity.

Emissivity of surfaces in the infrared range of the spectrum We present examples of the emissivity of different underlying surfaces in the infrared range of the spectrum. The most detailed data on these characteristics are available for ‘transparency windows’ of the atmosphere which is used in satellite meteorology to determine the temperature of the underlying surfaces (Chapter 10). Table 6.3. gives the emissivity ε in the spectral range 8–14 µm [45]. Recently, a large number of theoretical and experimental investigations have been carried out for determining the spectral dependence of the emissivity of different surfaces in the infrared region of the spectrum. As an example, Fig.6.11 gives the spectral dependences of the emissivity of the water surface, measured (at different wind speeds in the vicinity of the water surface) and calculated using the model identical with that described previously in subsection ‘Theoretical models’ [108]. The figure indicates that the experimental and calculated results are in relatively good agreement. At present, there are special databanks for the spectral dependences of the emissivity properties of different surfaces, obtained by experimental methods. These data are used for modelling radiation fields in the atmosphere–underlying surface system and also for remote measurement of the characteristics of the underlying surfaces, especially their temperature. Table 6.3. Emissivity of different surfaces in the infrared range Surface

ε

Surface

ε

Granite

0.898

Clay

Bazalt

0.934

Asphalt

0.956

Dolomite

0.958

Grass, dense cover

0.976

Sandstone

0.935–0.985

Snow

0.99

0.943

Water

0.98–0.993

Gravel

0.963–0.968

Sand, quartz, dry

0.914

Water with thin oil film

0.954–0.972

Sand, wet

0.934

Concrete

0.942–0.966

Black sol

0.965

Water with machine oil film

Loam

0.98

276

0.960

Emissivity,

ε

Optical Properties of Underlying Surfaces

Wave number cm –1 Fig.6.11. Measured (1,2,3) and calculated (4) spectral dependences of the emissivity of the water surface in the infrared range of the spectrum [108].

Emissivity of surfaces in the microwave range of spectrum The main special feature of the emissivity of the natural surfaces in the microwave range of the spectrum is their considerable variability in comparison with the infrared range of the spectrum. For example, the emissivity of the water surface is in the range 0.2–0.95 depending on its temperature, salt content, the velocity of driving wind, the condition of the surface and the observation angle. It also depends on the polarization characteristics of radiation (see, for example, Fig. 6.6. showing the coefficient of reflection of the smooth sea surface for a wavelength of 3 cm). On the other hand, a number of dry land surfaces have the emissivity close to unity (Table 6.4) [45, 119]. To describe the emissivity of the surface with waves, various researchers developed a number of semi-empirical models taking into account both the process of formation of waves and the possible presence of foam formation on the surface arising in disruption of the waves [85, 119]. According to the data by various authors, the process of breakage of the waves starts at wind speeds of 3–7 m/s. The determination of the relationship between the emissivity of the sea surface and, for example the velocity of the driving wind enables remote methods of determination of the velocity to be developed.

277

Theoretical Fundamentals of Atmospheric Optics Table 6.4. Emissivity of different surfaces at λ = 3.2 cm

Surface Soil (sample)

Air temperature, °C

Description Layer thickness 20 cm, Moisture content 6.4% Moisture content 19.5%

Emission coefficient

21 20

0.947 0.919

Soil

Moisture content on surface 21%, at a depth of 20 cm 17.5%

23

0.923

"

Moisture content on surface 34.6% at a depth of 20 cm 17.5%

22.5

0.668

Frozen soil (sample)

Temperature 0.4 °C Moisture content 13.9%

1.2

0.923

Defrosted soil

Temperature 0.4 °C 0

0.892

Snow cover

Moisture content 14% Snow on soil, layer thickness 20–30 cm density 0.408 g/cm 2

1.2

0.956

Frozen soil

Thickness of frozen layer 11–12 cm, moisture content at surface 45.6%

0.4

0.941

Peat

Moisture content 114% Moisture content 159%

18 15

0.943 0.918

Clay

Moisture content 9%

15

0.902

Plywood

Sheet layer thickness 8 mm

21

0.829

"

Sheet covered with aluminium paint

23

0.728

Concrete covered with snow

30 cm thick plate snow thickness 8–10 cm

0

0.906

0

0.669

Motorway

15

0.974

Wet concrete Asphalt

278

CHAPTER 7

FUNDAMENTALS OF THE THEORY OF TRANSFER OF ATMOSPHERIC RADIATION

Atmospheric radiation In Chapter 3 we examined different types of atmospheric radiation. We introduced important concepts of thermal (equilibrium) radiation and described its main laws. Formally speaking, all other types of generation of radiation of the atmosphere may be related to another large class – non-equilibrium radiation of the atmosphere. At present, it is accepted to subdivide the non-equilibrium radiation into non-equilibrium infrared radiation and glow of the atmosphere. This division is based on the spectral principle – nonequilibrium radiation in the ultraviolet, visible and near infrared ranges of the spectrum is related to the glow of the atmosphere, and non-equilibrium radiation in the middle and far infrared range – to non-equilibrium infrared radiation. Another difference is that the main mechanisms of formation of the glow of the atmosphere and non-equilibrium infrared radiation differ. The glow of the atmosphere is caused in most cases by the processes of excitation of electronic states of the molecules and atoms as a result of the adsorption of high energy radiation of the Sun in the ultraviolet and visible ranges and the energy of fluxes of different particles. Nonequilibrium infrared radiation forms because of the disruption of local thermodynamic equilibrium (LTE) in the upper layers of the atmosphere. In this case, the formation of non-equilibrium radiation is caused by the relatively small number of collisions of the molecules (transferring the system in the lower layers of the atmosphere to the equilibrium state), the non-isothermal nature of the atmosphere and the losses of the energy of the system as a result of the radiation of the atmosphere outgoing into the space. However, the absorption of the solar radiation arriving in the atmosphere is also an important factor in this case. In this chapter, special attention is given to the fundamentals of 279

Theoretical Fundamentals of Atmospheric Optics

the theory of transfer of the thermal radiation of the atmosphere [20, 34, 37, 91, 103]. In the final paragraphs of the chapter we examine briefly the non-equilibrium infrared radiation and glow of the atmosphere.

7.1. Transfer of thermal radiation In chapter 3 we derived equation (3.4.28) for the intensity of monochromatic natural thermal radiation I ν ↑ (z) at arbitrary altitude z. Using this relationship, we can write the following expression for the intensity of rising radiation at altitude z:

∫

z   = I ν ,0 exp  − sec θ kν ( z′) dz′  +   0   z z   + sec θ kν ( z′) Bν [T ( z′)]exp  − sec θ kν ( z′′)dz′′  dz′.   z′ 0  

I ν↑ ( z , θ)

∫

∫

(7.1.1)

Here I ν,0 is the radiation of the underlying surface which must be determined separately. This expression corresponds to upward radiation (i.e. the radiation into the upper atmosphere) for the model of the plane-parallel horizontally homogeneous atmosphere. Similarly, we can write an expression for the intensity of monochromatic downward thermal radiation for the same model of the atmosphere:

∫

∞   I ν↓ ( z , θ) = I ν ,∞ exp  − sec θ kν ( z′)dz′  +   z   ∞ z′   + sec θ kν ( z′) Bν [T ( z′)]exp  − sec θ kν ( z′′)dz′′  dz′,   z z  

∫

∫

(7.1.2)

where I ν,∞ is the intensity of radiation at the upper boundary of the atmosphere. Usually, for the infrared range of the spectrum this radiation is assumed to be equal to zero. For the microwave range of the spectrum in equation (7.1.2) we must take into account the relict microwave radiation arriving from space. Its brightness temperature is at present assumed to be equal to 2.7 K. In the equations (7.1.1) and (7.1.2) there is the transmittance function of the atmospheric layer (z 1 , z 2 ):

280

Fundamentals of the Theory of Transfer of Atmospheric Radiation z2   Pν (θ, z1 , z2 ) = exp  − sec θ kν ( z′)dz′  .   z1  

∫

(7.1.3)

Using these transmittance functions, the expressions (7.1.1) and (7.1.2) can be written in a more compact form:

∫ 0

dPν ( z′, z ) dz′, dz′

∫

dPν ( z , z′) dz′. dz′

I ν↑ ( z , θ) = I ν ,0 Pν (0, z ) + Bν [T ( z′)] z

∞

I ν↓ ( z , θ) = I ν ,∞ Pν ( z , ∞) − Bν [T ( z′)] z

(7.1.4)

(7.1.5)

Further, in this chapter we examine in greater detail the methods of determining the transmittance functions of the atmosphere. Radiation of the underlying surface in expression (7.1.1) and (7.1.4) is often represented by the emissivity of the surface ε (see for more details chapter 6), i.e. I ν,0 = ε ν B ν (T 0 ),

(7.1.6)

where T 0 is the surface temperature. However, the difference of the emissivity of the surface from unity assumes the existence of reflection of downward thermal radiation of the atmosphere from the surface. The intensity of reflection may be written in different form depending on the given reflection model (see chapter 6). In the simplest case of mirror reflection it can be presented in the form I ν (θ) = (1 – ε ν ) I ν ↓ (0),

(7.1.7)

where I ν ↓ (0) is the intensity of downward thermal radiation. Thus, I ν,

0

=

ε ν B ν (T 0 ) + (1 – ε ν ) I ν ↓ (0).

In various practical problems of atmospheric physics we have not been interested in many cases in the transfer of monochromatic radiation (with the exception of, for example, theoretical study of special features of radiation transfer or in the propagation or ‘almost’ monochromatic radiation of lasers). In fact, calculating, for example, the values of the radiation influxes of heat for determining the variation of the atmospheric temperature as a result of radiation

281

Theoretical Fundamentals of Atmospheric Optics

heat exchange, it is necessary to integrate the appropriate influxes of radiation in respect of frequency (or wavelength). In the same manner, analysing the results of measurements of specific characteristics of the radiation field, we examine the radiation in finite spectral ranges. To obtain the intensity of thermal radiation in finite spectral ranges it is necessary to integrate the monochromatic intensities in respect of frequency (or wavelength) I ∆ν =

∫ I d ν,

∆ν

ν

(7.1.8)

where I ∆ν is the intensity in the spectral range ∆ν. Substituting into equation (7.1.8), for example, equation (7.1.4) and ignoring the dependence on θ, we obtain the intensity of upward radiation in finite spectral range ∆ν: I ∆ν ( z ) =

+

∫I

∆ν

ν ,0 Pν (0, z ) d ν +

∫ ∫ B [T (z ′)] z

∆v 0

v

dPv ( z ′, z ) dz ′dv. dz ′

(7.1.9)

Equation (7.1.9) or identical expressions for downward radiation can be used to calculate the intensity of thermal radiation in finite spectral ranges but it can also be greatly simplified. For this purpose, it is necessary to take into account the special features of the spectral behaviour of the Planck function and the monochromatic transmittance functions present in all members of the equation (7.1.9). As shown in Chapter 4 (Fig.4.6), the monochromatic transmittance functions are very rapidly changing functions of frequency (or wavelength). At the same time, the Planck function changes quite slowly with frequency. Therefore, if we examine relatively narrow spectral ranges (~50–100 cm –1 ), in which the variation of the Planck function can be ignored, we can write an approximate equation for the intensity of thermal radiation in finite spectral ranges. For upward radiation, for example ↑ I ∆ν ( z ) = I ν ,0

∫ 0

∫

ν

dPν ( z′, z ) dz′d ν, dz′ ∆ν

+ Bν [T ( z′)] z

∫ P (0, z )d ν +

∆ν

282

(7.1.10)

Fundamentals of the Theory of Transfer of Atmospheric Radiation

where B ν_ [T(z´)] and I ν,_ 0 are the Planck function and the contribution of the underlying surface at some mean frequency ¯v of the examined spectral range. The integrals in respect of frequency present in equation (7.1.10) determine the transmittance functions and the derivatives for the finite spectral ranges. Therefore, expression (7.1.10) can be written in a new form

I ∆↑v ( z ) = I v ,0 Pv (0, z )∆v +

∫ z

0

∫

dP∆v ( z ′, z) dz ′. dz ′ ∆v

+∆v Bv [T ( z ′ )]

(7.1.11)

In particular it should be mentioned that the assumption on the weak dependence of the Planck function of frequency enables us to write the equation for the intensity of thermal radiation in finite spectral ranges in the same form as for monochromatic intensity. The difference between the equations (7.1.4) and (7.1.11) is that equation (7.1.11) contains the functions of transmittance for finite spectral ranges in contrast to the monochromatic functions of transmittance in equation (7.1.4). This approximation reduces the problem of integration of monochromatic radiation (equation (7.1.8)) to the problem of obtaining the transmittance functions for finite spectral ranges. Naturally, similar equations can also be obtained for downward thermal radiation.

7.2. Transmittance functions of atmospheric gases The transmittance functions of the atmosphere are of fundamental importance in atmospheric optics. This is due to the fact that they are used for solving various direct problems of atmospheric optics - calculations of intensities, fluxes and influxes of radiation. They are also used for interpreting the measured results, for example, when using different remote methods of measurements of parameters of the atmosphere and the surface.

Determination of the transmittance function We now return to describing the process of attenuation of radiation in the atmosphere of the planets. In accordance with the Bouguer’s law (equation (3.4.5)) the monochromatic intensity of radiation, transmitted through the homogeneous layer of the atmosphere, containing amount of the absorbing substance u = ρl, is equal to 283

Theoretical Fundamentals of Atmospheric Optics

I ν (u) = (I 0 , ν exp (–k v u),

(7.2.1)

where in the present case k ν is the mass molecular absorption coefficient. If the weakening coefficient is determined by molecular absorption in the multicomponent gas atmosphere, then in accordance with the equations (4.4.1) and (4.4.3) given previously, the relationship (7.2.1) may be written in the new form  I ν (u ) = I 0,ν exp  −  

∑∑ S j

ij (T ) f ij ( p, T , ν − ν 0,ij ) u j

i

 ,  

(7.2.2)

where U j is the content of the absorbing j-th gas. The relationship Pν (u ) =

 I ν (u ) = exp  −  I 0,ν 

∑∑ S j

ij (T ) f ij ( p , T , ν − ν 0,ij )u j

i

   

(7.2.3)

is the monochromatic transmittance function for molecular absorption. The transmittance function is expressed either in percent or in fractions of unity. The case P ν =1 (at 100%) corresponds to the absorption, in the case P ν = 0 to the complete absorption of radiation on the examined path. Equation (7.2.3) describes absorption on a homogeneous path containing j absorbers (different gases), i.e. on the path characterised by constant pressure p and temperature T. (Strictly speaking, at constant partial pressures of the absorbing gases p j and constant pressure of secondary gases). Horizontal paths along the surface of the Earth 5–10 km long are a good approximation of homogeneous paths. In a general case when radiation propagates on vertical (or inclined) paths, we are concerned with the propagation of radiation in media with pressure and temperature changing from point to point. In this case, equation (7.2.3) is generalized for the monochromatic transmittance function of a inhomogeneous medium as follows:  Pν (l ) = exp  −  

∑∑ ∫ S l

j

i

  

ij (T )(l )) × f ij ( p (l ), T (l ), ν − ν 0,ij ) du j (l )  .

0

(7.2.4)

These monochromatic transmittance functions are also found in equation (7.1.4). The differential of the content of the absorbing gases du may be presented in different forms depending on the characteristics of the gas content in the atmosphere used in this case. For molecular absorption, it is expressed through the density of the absorbing gas ρ j : 284

Fundamentals of the Theory of Transfer of Atmospheric Radiation

du j (l) = p j (l)dl.

(7.2.5)

As mentioned previously, for practical problems it is interesting to use different quantities (for example, radiation intensity) calculated for finite spectral ranges. In this case, as followed from equation (7.1.9), we are concerned with the transmittance functions (over the derivatives) in finite spectral ranges. For example, let us assume that our device records the solar radiation passing in the atmosphere along the path l. Consequently, the signal recorded by the device according to (7.2.1) and (7.2.4) can be written in the form I ∆ν (ν , l ) =  exp  −  

∫ ϕ(ν − ν ') I (ν ') × 0

∆ν

∑∑ ∫

 kij (ν ', p (l ′), T (l ′))ρ j (l ′)dl ′  d ν ',  0 

(7.2.6)

l

j

i

where ∆ν is the frequency interval determined by the spectral resolution of the device; ν is the interval to wich the measured intensity is assigned; I 0 (ν') is the intensity of solar radiation at the upper boundary of the atmosphere; ϕ( ν –ν´) is the slit function of the device characterizing the sensitivity of the device to radiation at different frequencies inside the interval ∆ν. (Strictly speaking, the device integrates radiation not only in respect of frequency but also the solid angle and measurement time but to simplify the considerations this is not taken into account in equation (7.2.6)). As in (7.2.3) and (7.2.4), we can introduce the concept of the transmittance functions for finite spectral ranges. For example, for a inhomogeneous medium, we have P∆ν ( ν, l ) =

∫

I ∆ν (ν ', l ) = I ∆ν (ν , o)

∑∑ ∫

(7.2.7)

l   ϕ(ν − ν ') I 0 (ν ') exp  − kij (ν ' p (l ′), T (l ′))ρ,(l ′)dl ′  d ν  j i  0   . = ∆ν ϕ(ν − ν ') I 0 (ν ')d ν '

∫

∆ν

285

Theoretical Fundamentals of Atmospheric Optics

If it is assumed that in the examined spectral interval ∆ν the radiation incident on the layer is not selective, i.e. I 0 (ν) = const, taking into account the standard normalisation of the slit function of the device

∫ ϕ(ν − ν ')d ν ' = ∆ν

∆ν

(7.2.8)

we obtain from (7.2.7) a simple expression for the transmittance function often used in atmospheric optics: P∆ν ( ν, l ) =

 × exp  −  

∑∑ ∫ k l

j

i

0

∫

1 ϕ(ν − ν ') × ∆ν ∆ν

ij (ν ',

 p (l ′), T (l ′))ρ j (l ′)dl ′  d ν '.  

(7.2.9)

Naturally, in calculations of radiation energy, for example, radiation fluxes in the atmosphere, the slit function is not present or, formally, it is equal to unity. On the other hand, in analysing the results of measurements of different characteristics of the radiation fields, it is important to take into account the slit function.

7.3. Methods of determination of transmittance functions Because of the extensive use of transmittance functions in the atmospheric physics, special attention is being paid to their determination. Various experimental and calculation methods of determination have been developed. The most important are [34]: – direct calculation method; – method of the models of absorption bands; – method of integration in respect of the absorption coefficient; – experimental methods (laboratory and field size).

Direct method of calculating transmittance functions If information on the parameters of the fine structure (position, intensity, line half width, etc) for different absorption bands is available, the transmittance functions can be calculated from the equations presented previously. To determine the transmittance functions in finite spectral intervals, this method requires integration in respect of the frequency range of the monochromatic 286

Fundamentals of the Theory of Transfer of Atmospheric Radiation

transmittance functions. This approach is referred to as the line– by–line method of calculating transmittance functions. The advantages of the approach are: 1. The possibility of obtaining the transmittance functions for homogeneous and inhomogeneous media for different geometries of the propagation of radiation for any mixtures of gases, arbitrary slit functions of spectral devices, etc. In this sense, the direct method is universal. Of course, in this case it is necessary to have information not only on all parameters of the fine structure, describing molecular absorption, but also on their dependences on the parameters of the physical state of the medium – temperature, pressure, etc. 2. The highest potential accuracy because the direct method does not require any simplifications (for example, approximated consideration of the inhomogeneity of the atmosphere, etc). The actual accuracy of the method is determined by the accuracy of the initial information on the parameters of the fine structure and the accuracy of defining these functions dependencies on the parameters of the physical state of the medium. The main shortcoming of the direct method is the need to carry out a large volume (even on the scale of currently available computers) of calculations. In this case, the considerable computing time is required in the calculation of the monochromatic coefficients of absorption and numerical integration of the monochromatic transmittance in respect of the frequency. If it is taken into account that the operation of integration should be carried out with strict approach for every new model of the atmosphere, the path of propagation of radiation, the slit function of the device, etc, then it is understandable why in addition to the direct method many problems of atmospheric physics are also solved using other methods of determination of the transmittance functions.

The method of the models of absorption bands The concept is based on the replacement of the actual spectral structure of the absorption bands by a specific analytical or statistical model of the mutual position of the absorption lines, distribution of their intensities, etc [20, 91, 102]. The introduction of these approximations makes it possible to integrate the monochromatic functions of transmittance in the analytical manner in respect of frequency in many cases. It is important that the analytical expressions for the transmittance functions in the

287

Theoretical Fundamentals of Atmospheric Optics

modelling approach depend on a small number of parameters, sometimes 2–3. Because of the considerable saving of computer resources, the operating speed of this method is several times orders of magnitude higher than that of the direct method of calculations. Naturally, the parameterization of the absorptions spectra leads to additional errors in the resultant values of the transmittances functions. It is therefore necessary to ensure optimum selection of the model of absorption band for a specific spectral range or spectral interval and to analyse the accuracy of the approach. When using the absorption models, attention is often given to the current data on the parameters of the fine structure of the specific investigated model for determination of the model parameters. In a number of cases, the expressions for the transmittance functions, obtained within a specific modelling approach, are used as approximations for the transmittance functions, obtained in experiments or in direct calculations. A large number of different models of absorption bands have been proposed: the model of isolated lines, the Elsasser model (regular model), various random models, quasi-random Plass model and many others. A detailed review and analysis of different models of the bands were published in, for example, monographs [20, 37, 91]. We present only the most frequently used models of absorption bands.

Model of the isolated line The simplest model of the absorption band is the model of the isolated spectral line. If it assumed that the main factor of broadening the line is collisions and the appropriate line shape is of the Lorentz type, then we have P∆ν =

∫

 1  1 S α Lu exp  − d ν. 2 2  ∆ν ∆ν  π (ν − ν 0 ) + α L 

(7.3.1)

As indicated by the name of the method, it is assumed that only one spectral line is found in the interval ∆ν. Equation (7.3.1) is converted. We introduce new variables:

x=

ν − ν0 α Su , y= L, z= . ∆ν ∆ν 2πα L

(7.3.2)

It is assumed that the examined line is isolated, i.e. does not overlap with any other lines so that we can extend the range of integration 288

Fundamentals of the Theory of Transfer of Atmospheric Radiation

from –∞ to +∞. We introduce the absorption function

A∆ν = 1 − P∆ν =

∫

 1  S α Lu 1  d ν. 1 − exp  − 2 2   ∆ν ∆ν   π (ν − ν 0 ) + α L  

Consequently for the absorption function in the interval ∆ν we can write

A∆ν =

∫

  2 zy 2   dx. 1 − exp  − 2 2    x + y  −∞  +∞

(7.3.3)

The integral (7.3.3) for the absorption functions A ∆ν is expressed through the special functions – cylindrical functions of the first kind referred to also as Bessel functions: A ∆ν = 2πyze –z (J 0 (iz) – iJ 1 (iz)) = 2πyL(z),

(7.3.4)

where J 0 (iz) and J 1 (iz) are the zero and first Bessel function of the purely imaginary argument. The function L(z) = ze – z (J 0 (iz) – iJ 1 (iz))

(7.3.5)

is often referred to as the Ladenburg–Reiche function. Examining the asymptotic behaviour of the function L(z), we can obtain important approximations for the absorption in the isolated line. At low values of z, the term e –z and J 0 (iz) in (7.3.5) tend to unity, and J 1 (iz) to zero. Consequently, L(z) ~z and we have

A∆ν = 2πyz =

Su . ∆ν

(7.3.6)

The low values of z correspond to the low values of the product Su, i.e. the case of weak absorption. This case is also referred to as the region of linear absorption as a result of the linear dependence of A ∆ν on u. It should be mentioned that in this limiting case absorption does not depend on pressure. The law of linear absorption can also be determined directly from equation (7.3.1.). It is written in the following form P∆ν =

∫

1 exp(− k (ν )u ) d ν. ∆ν ∆ν

It is assumed that k(ν)u is very small even in the centre of the 289

Theoretical Fundamentals of Atmospheric Optics

absorption line (ν = ν 0 ). Consequently, expanding the exponent into a series (e x = 1 + x + …) and retaining only the first two members of the expansion, we have P∆ν =

∫

∫

∫

1 1 u k (ν )u d ν = 1 − k (ν ) d ν. (1 − k )(ν )u ) d ν = 1 − ∆ν ∆ν ∆ν ∆ν ∆ν ∆ν

Passing as previously to the infinite limits of integration in respect of frequency and taking into account normalization (4.4.4) we get

A∆ν =

Su ∆ν

It should be mentioned that this conclusion shows that the law of linear absorption at low values of Su holds for any and not only for the Lorentz contour of the absorption line. At high values of z we use the well-known asymptotic equation for the Bessel function J m ( x) =

Consequently

mπ π  2  sin  x − +  at x → ∞. 2 4 πx 

L ( z ) = ze − z J 0 (iz ) − iJ1 (iz) =

 2 2 2 2 2 sin(iz ) = ze− z  + cos(iz) − i sin(iz) + i cos(iz) = ( 2 2 2 2   πiz

=

2z − z 2 e 2 π

−i ( (1 − i )

ei (iz ) − e − i (iz ) ei (iz ) − e − i (iz )  + (1 + i )  2i 2 

Ignoring terms e –z below the modulus, we obtain L( z ) = =

2z 2 π 2

2z − z z e e π

−i (1 + i ) =

−i

i −1+ i −1 = 2i

2z 2  2 2 −i (1 + i ) =  π 2  2 2  290

2z . π

Fundamentals of the Theory of Transfer of Atmospheric Radiation

Consequently, 2 z 2 Suα L = . π ∆ν

A∆ν = 2πy

(7.3.7)

The equation (7.3.7) corresponds to the case of strong absorption or the square root law. The latter name is associated with the fact that the dependence of absorption on the amount of absorbing matter and pressure (it should be mentioned that α L is proportional to p) is presented as up . It is also important to mention that in the case of strong absorption in the centre of the line (when P ν0 = 0) the variation of absorption A ∆ν (u) takes place as a result of the variation of the extent of absorption only in the ‘wings’ of the isolated spectral line. This holds not only for the Lorentz but also Voigt line because the wings of the Voigt line are determined mainly by the Lorentz line shape. The examined model of the isolated line is used for upper layers of the atmosphere where the spectral lines are very narrow and may be regarded as isolated.

Elsasser model Analysis of the absorption spectra of different linear molecules shows the presence of a regular structure in the arrangement of the lines corresponding to the same vibration transition, and the intensities of these lines change only very slowly. Therefore, the regular model of the absorption band, or the Elsasser model, has been proposed, Fig.7.1. For a regular model consisting of an infinite sequence of identical spectral Lorentz lines, situated at distance ∆ν from each other, the absorption coefficient can be written in the form: k (ν ) =

∑ π (ν − n∆ν)

n =+∞

SαL

1

n =−∞

2

+ α 2L

.

(7.3.8)

Passing in (7.3.8) to the absorption function using the variable x, y, z introduced for the model of the isolated line, we obtain

A∆ν =

∫

∑

  n=+∞  2 zy 2 dx, 1 − exp  − 2 2    n=−∞ ( x − n) + y   −1/ 2  +1/ 2

where the integration limits correspond to the range [ν 0 –∆ν/ 2,ν 0 +∆ν/2]. For the sum in the exponent we have the analytical 291

Theoretical Fundamentals of Atmospheric Optics

Fig.7.1. Elsasser model [20].

expression (obtained from the theory of Loran series). Consequently we obtain A ∆ ν = 1 – E(y, z), where E(y,z) is the Elsasser function

E ( y, z) =

∫

 2 πyzsh2 πy  exp  − dx.  ch2 πy − cos2 πx  −1/ 2 +1/ 2

(7.3.9)

It should be mentioned that the hyperbolic sinuses and cosines are determined as follows

1 1 shx = (e x − e− x ), chx = (e x + e− x ). 2 2

(7.3.10)

Let it be that y→∞, and, therefore, according to (7.3.10), sh2 πy → 1 ch2πy

and from (7.3.9) we obtain

 Su  A∆ν = 1 − exp(−2πyz ) = 1 − exp  − .  ∆ν 

(7.3.11)

The examined case corresponds to the situation when α L >>∆ν, i.e. the lines are very close to each other and greatly overlap. Here, as indicated by (7.3.11), the transmittance function depends exponentially on u, as in the case of monochromatic radiation, and there is no spectral dependence of transmittance. It should also be mentioned that the transmittance function does not depend on pressure. The examined situation is actually observed when the half width of the lines is considerably greater than the distance between them, for example, at high pressures (it should be mentioned that α L is proportional to p). These special features of absorption in this case are characteristic of all models of the absorption bands. In 292

Fundamentals of the Theory of Transfer of Atmospheric Radiation

particular, they are found (with different degrees of approximation) in electronic absorption bands, and also in infrared spectra of heavy molecules consisting of very closely distributed absorption lines. If parameter y is small (y → 0) then, as shown directly from (7.3.9), (7.3.10), the Elsasser function changes to the previously examined model of the isolated line. The condition y 0

I ( τ0 , η, η0 , ϕ) = 0 if η < 0.

(8.1.11)

The relationships (8.1.10)–(8.1.11) are the final form of the transfer equation of scattered solar radiation in the plane-parallel atmosphere [65].

Function of sources. Single and multiple scattering The contribution to scattering to the intensity (8.1.9) is controlled by the two last terms in the right-hand part. As shown in Chapter 3, this contribution is identical to the presence of additional light sources in the medium. We introduce the function of sources B (τ,η,η 0 ,ϕ) determining it as: 339

Theoretical Fundamentals of Atmospheric Optics

B ( τ, η, η0 , ϕ ) =

+

∫ ∫

Λ (τ) d ϕ′ x(τ, ω) I (τ, η′, η0 , ϕ′) d η′ + 4π 0 −1 2π

1

Λ(τ) Sx(τ, ω0 )exp(−τ η0 ). π

(8.1.12)

The integrated term in (8.1.12) is associated with the scattering of direct solar radiation. This scattering, i.e., the scattering of direct radiation, is single scattering. The integral term in (8.1.12) is associated with the scattering of the already scattered radiation I(τ,η′,η 0,ϕ′) and this scattering is multiple scattering. * The physical meaning of these terms is relatively simple: The term outside the integral is the direct solar radiation transmitted to the level τ, taking into account the fraction of scattering in the general extinction Λ(τ) (this leads to the term ‘the albedo of single scattering’!), and the phase function – ‘the strength’ of scattering at specific angle ω 0 . Integral term – similar consideration of the contribution of the scattered radiation travel to the level τ from all possible directions. After introducing the function B (τ,η,η 0 ,ϕ) the transfer equation has the form

η

I (τ, η, η0 , ϕ) = − I (τ, η, η0 , ϕ) + B(τ, η, η0 , ϕ). dτ

(8.1.13)

However, (8.1.13) is the linear differential equation for the intensity I(τ,η,η 0 ,ϕ) of the type dy/dx = a(x) y(x) + b(x), whose general solution (this has already been discussed in Chapter 3) is

∫

∫

∫

x  x x  y ( x ) = y ( x0 ) exp  a ( x′) dx′  + b ( x′) exp  a ( x′′) dx′′  . x  x x    0

0

*It should be mentioned that the concept ‘single scattering’ and ‘multiple scattering’ are used, in addition to transfer theory, in the optics of aerosols in a completely different sense. This often causes confusion and misunderstanding. In the optics of aerosols ‘single scattering’ denotes the scattering on a particle described regardless of the presence of other particles, for example, as in Mie theory. ‘Multiple scattering’ is the combined diffraction of electromagnetic waves on several particles. In transfer theory ‘multiple scattering’ denotes consideration of the radiation scattered several times during its passing through the atmosphere. It may be seen that the concepts are completely different. It should be mentioned that usually the volume coefficients of aerosol scattering are calculated in the approximation of single scattering in the sense of aerosol optics but used for calculating multiple-scattered radiation in transfer theory.

340

Main Concepts of the Theory of Solar Radiation Transfer

1 1 Here a( x ) = − , b( x) = B (τ, η, η0 , ϕ), the integration variable – η η the optical thickness τ, the initial value x 0 is 0 for the direction from the upper boundary (η > 0) and τ 0 for the direction from the lower boundary (η< 0), the values of y(x 0 ), according to (8.1.11) are equal to zero. Thus, we have

I (τ, η, η0 , ϕ) =

∫ τ

S B(τ′, η, η0 , ϕ) × 4η 0

 τ − τ′  × exp  −  d τ ', η  

I (τ, η, η0 , ϕ) = −

if η > 0,

(8.1.14)

∫ τ0

S B (τ′, η, η0 , ϕ) × 4η τ

 τ − τ′  × exp  −  d τ′, η  

if η < 0.

Finally, equations (8.1.14) are not solutions of the transfer equation because in a general case the function of sources B (τ,η,η 0 ,ϕ) according to (8.1.12) depends on intensity. However, they do provide an explicit expression for the required intensity through the function of sources and this is very important. For example, from (8.1.14) we can immediately write a general solution of the transfer equation in the approximation of single scattering, i.e., when the function of sources takes into account the term outside the integral B(τ, η, η0 , ϕ) = I (τ, η, η0 , ϕ) = −

Λ(τ) Sx(τ, ω0 )exp(− τ η0 ) : 4

∫ τ

S Λ(τ′) x(τ′, ω0 ) × 4η 0

 τ′ τ − τ′  × exp  − −  d τ′, η   η0 I ( τ, η, η0 , ϕ) = −

if η > 0,

∫ τ

S Λ (τ′) x(τ′, ω0 ) × 4η τ 341

0

(8.1.15)

Theoretical Fundamentals of Atmospheric Optics

 τ′ τ − τ  × exp  − −  d τ′, η   η0

if η < 0.

The approximation of single scattering (8.1.15) is often used in problems where the high accuracy of calculating scattered radiation is not required.

Integral equation for the functions of sources Let us substitute the equations for intensity (8.1.14) in the definition of the function of sources (8.1.12). We obtain:

∫

Λ(τ) B (τ, η, η0 , ϕ) = d ϕ′ × 4π 0 2π

∫

∫

τ 1  τ − τ′  d η′ ×  x ( τ, ω) B(τ′, η′, η0 , ϕ′)exp  −  d τ′ − η′ 0  η′  0

∫

− x ( τ, ω) 0

−1

(8.1.16)

∫

τ   τ − τ′  d τ′ B (τ′, η′, η0 , ϕ′) exp  − d τ′  +  η′ τ  η′   0

+

 τ Λ(τ) Sx(τ, ω0 )exp  −  . 4  η0 

(8.1.16) contains only the function B (τ,η,η 0 ,ϕ), and consequently we obtained an equation for the function of sources whose solution is related to the required intensity by the simple relationships (8.1.14).Regardless of the cumbersome form, from the mathematical viewpoint equation (8.1.16) is more convenient than the equation for intensity (8.1.9) because it is an integral equation (not integro-differential). Therefore, in the theory of transfer we usually concerned with (8.1.16) [47, 65]. The resultant equation (8.1.16) is the Fredholm integral equation of the second kind. The mathematical theory of these equations has been developed quite sufficiently, in particular, the existence and uniqueness of the solution for these equations have been shown. In the ‘operator ’ form, equation (8.1.16) can be written in the form [46, 53]

B = KB + q , 342

Main Concepts of the Theory of Solar Radiation Transfer

Where B is the sought function of sources B (τ,η,η 0 ,ϕ); K is the integral operator of scattering with a kernel  Λ ( τ)  τ − τ′  x( τ, ω) exp  −  , ′ ′ πη η 4     0 ≤ τ′ ≤ τ, 0 ≤ η′ ≤ 1, K (τ, η, ϕ, τ′, η′, ϕ′) =   − Λ (τ) x(τ, ω) exp  − τ − τ′  ,    4πη′  η′    τ ≤ τ′ ≤ τ0 , −1 ≤ η′ ≤ 0, q is a free term;

q (τ, η, η0 , ϕ) =

Λ (τ) Sx( τ, ω0 )exp( − τ η0 ). 4

The formal solution of the Fredholm equation of the second kind is the Neumann series

B = q + Kq + K 2 q + K 3 q + ...

(8.1.17)

The terms of the series have a simple physical meaning: the first (q), as already explained, corresponds to the contribution of singlescattered light; the second (Kq) – the application of the scattering operator to single-scattered light, i.e. the contribution of twicescattered light; similarly, the third one (K 2 q) = K(Kq) – the contribution of light scattered by three times, etc. This means that the Neumann series (8.1.17) is the expansion of the contribution of scattered light in respect of the multiplicity of scattering. It should be mentioned that the kernel K and the free term q are directly proportional to the albedo of single scattering Λ(τ). Let it be that Λ is independent of τ. Then, at the n-th term of the series (8.1.17) we have the coefficient Λ n which evidently determines the rate of convergence: as Λ approaches unity, i.e. as the absorption decreases in comparison with scattering, the rate of convergence of the series decreases and the multiplicity of scattering which must be taken into account in the calculations, increases. Since q is directly proportional to the parameter S and K is independent of S, equation (8.1.17) shows that the function of sources B and according to (8.1.14), the intensity of scattered radiation are directly proportional to S. Consequently, in complete correspondence with the physics of processes, the intensity of scattered light is 343

Theoretical Fundamentals of Atmospheric Optics

directly proportional to the flux at the upper boundary of the atmosphere. Therefore, to simplify considerations, in solving the transfer equation it is often assumed that S = 1 and the obtained intensity has been multiplied by the specific value of S.

8.2. Analytical methods in radiation transfer theory Expansion of scattering phase function into a series in terms of Legendre polynomials In the previous paragraph we obtained the main equations of radiation transfer taking multiple scattering into account. The most important part of the theory of radiation transfer is the mathematical analysis and obtaining solutions in both partial and general cases. The mathematical apparatus of transfer theory has been described in many monographs [47, 64, 65]. We present only the simplest examples of transformation of the transfer equation which are, however, very important for practical problems. The standard procedure of solving the differential and integral equations is the expansion of their parameters into a series in respect of orthogonal functions. For the transfer equation (8.1.16) we can achieve simplification in expansion of the scattering phase function into a series in terms of the Legendre polynomials. Therefore, the Legendre polynomials will be discussed briefly.* The Legendre polynomials P n (x) are determined by the equation 2 1 d ( x − 1) . Pn ( x ) = n 2 n! dx n n

(8.2.1)

For calculations in practice we can use the recurrent relationship

Pn ( x ) =

2n − 1 n −1 xPn−1 ( x ) − Pn−2 ( x ), n n

(8.2.2)

(where P 0 (x) = 1, P 1 (x) = x), which enables us to calculate in

succession P n , from (8.2.2): P2 ( x ) = (3 x 2 − 1), P3 ( x ) = (5 x 3 − 3 x ) . The 1 2

1 2

Legendre polynomials form an orthogonal system of the functions in the range [–1, 1] (main property), and for them

*The Legendre polynomials, spherical functions, Legendre adjoint functions and the addition theorem are described in detail in, for example, a textbook in [63]. 344

Main Concepts of the Theory of Solar Radiation Transfer

∫ 1

−1

∫

Pn ( x) Pm ( x)dx = 0, if n ≠ m; Pn2 ( x)dx = 1

−1

2 . 2n + 1

Correspondingly, any function f(x) continuous in the range [–1, 1] can be expanded into a series in terms of the Legendre polynomials:

∑ c P ( x), ∞

f ( x) =

where ck =

k =0

k

k

∫

2k + 1 f ( x )Pk ( x )dx. 2 −1

(8.2.3)

1

(8.2.4)

Using (8.2.2) and (8.2.3) for the scattering phase function we obtain

∑ x P (ω),

x (ω) =

where xi =

∞

i

i =0

∫

i

2i + 1 x(ω)Pi (ω)d ω. 2 −1

(8.2.5)

1

It should be mentioned that x0 =

(8.2.6)

∫

1 x(ω)d ω, but this is the condition 2 −1 1

of normalisation of (3.3.13) taking into account the change of the angle to cosine. Consequently, in all cases x 0 = 1. The expansion coefficient x 1 is an important characteristic of the phase function: x1 =

∫

3 x(ω)ωdω. 2 −1 1

Since ω is the cosine of the scattering angle, x 1 /3 is the mean cosine of scattering for the given phase function. It characterises its stretching forward: as the mean cosine increases the phase function becomes more stretched. For practical calculations it is interesting to consider finite series, i.e. to cut off (8.2.5) at some number of terms N. Unfortunately, for the strongly elongated aerosol phase functions it is necessary to take into account tens of even hundreds of terms in (8.2.5). Therefore, in calculations in practice using expansion of the indicatrices into a series in terms 345

Theoretical Fundamentals of Atmospheric Optics

of the Legendre polynomials, we are faced with the problem of approximating the elongated indicatrices [53]. It should be mentioned that for a Rayleigh phase function

3 x(ω) = (1 + ω2 ) 4

directly

1 x(ω) = P0 + P2 (ω) . 2 In the transfer equation, the phase function is a function of the incident angles and scattered radiation. For similar relationships we can use the addition theorem according to which

(

Pi ηη′ +

= Pi (η) Pi (η′) + 2

(1 − η )(1 − (η′) ) cos(ϕ − ϕ′) = 2

2

∑ (i + m)! P i

(i − m )!

m =1

m

i

(η) Pi m (η′)cos m(ϕ − ϕ′),

(8.2.7)

where P m i (x) are the Legendre adjoint functions determined by the relationship Pi m ( x ) = (1 − x ) 2

m

d m Pi ( x ) . dx m

(8.2.8)

For practical calculations of the Legendre adjoint functions we can use the recurrent equations:

Pnm+1 ( x ) = −

2n + 1 xPnm ( x ) − n − m +1

n+m Pnm−1 ( x ), n − m +1

at 0 ≤ m ≤ n − 1,

Pnm+2 ( x ) = 2( m + 1)

x

1 − x2

−( n( n + 1) − m( m + 1)) Pnm ( x ),

(8.2.9)

Pnm+1 ( x ) − at 0 ≤ m ≤ n − 2,

where Pn0 ( x ) = Pn ( x ); P11 ( x ) = 1 − x 2 . Taking into account (8.2.7) the expression for the scattering phase function has the form:

346

Main Concepts of the Theory of Solar Radiation Transfer

x(ω) = +

∑ x P (η)P (η′) + N

i

i =0

∑ 2 x ∑ (i + m)! P N

(i − m )!

i

i

i=0

m

i

m=1

i

i

(η)Pi m (η′)cos m(ϕ − ϕ′).

In the double sum we group the terms with the same index m: with m = 1 there are terms at all i from 1 to N, with m = 2 – at all i from 2 to N, etc. This means that x(ω) = +2

∑ x P (η) P (η′) + N

1

i=0

i

i

∑ cos m(ϕ − ϕ′)∑ x (i + m)! P N

(i − m )!

N

m =1

i =m

or in compact form x(ω) = p 0 (η, η′) + 2

where p m (η, η′) =

1

∑p N

m =1

m

(i − m )!

i

i=m

(η)Pi m (η′)

(η, η′)cos m(ϕ − ϕ′),

∑ x (i + m)! P N

m

i

i

m

(η)Pi m (η′).

(8.2.10)

(8.2.11)

For the unknown intensity and the function of sources we write formally expansions identical to (8.2.10) I ( τ, η, η0 , ϕ) = I 0 ( τ, η, η0 ) +

+2

∑I N

m

m=1

(τ, η, η0 ) cos mϕ,

(8.2.12)

B(τ, η, η0 , ϕ) = B 0 (τ, η, η0 ) + +2

∑B N

m =1

m

( τ, η, η 0 ) cos mϕ,

(8.2.13)

where I m (τ,η,η 0 ) and B m (τ,η,η 0 ) are some functions to be determined, m = 0, …, N. Substituting (8.2.12), (8.2.13) into the transfer equation (8.1.13) and equating the terms with the same m,

347

Theoretical Fundamentals of Atmospheric Optics

we obtain

η

I m (τ, η, η0 ) = − I m (τ, η, η0 ) + B m (τ, η, η0 ). dτ

(8.2.14)

Let us substitute now (8.2.10), (8.2.12) and (8.2.13) into expression for the function of sources through intensity (8.1.12) and calculate in the obtained equation the integrals in respect of azimuth. The product of the zero terms is independent of the azimuth and the integral is equal to 2π. The other terms in re-multiplication of the series (8.2.10) for the phase function and (8.2.12) for the intensity give the integrals of the type:

∫

2π

∫

2π

cos m1 (ϕ − ϕ′)cos m2 ϕ′d ϕ′ = cos m1ϕ cos m1ϕ′ cos m2 ϕ′d ϕ′ +

0

0

∫

2π

+ sin m1ϕ sin m1ϕ′ cos m2 ϕ′dϕ′. 0

∫ cos m ϕ′ cos m ϕ′ dϕ′

2π

However,

1

0

to π if m 1 = m 2 and

2

is equal to zero if m 1 ≠m 2 , and equal

∫ sin m ϕ′ sin m ϕ′ dϕ′

2π

1

2

is equal to zero for all m 1

0

and m 2 . Thus, after re-multiplying the series and integration in respect of the azimuth, equation (8.1.12) retains only the terms with equal indices, and at the zero term there will be coefficient 2π, and the remaining terms 4π cos mϕ. Equating now the terms with the same m in the right and left parts, we obtain Λ (τ) (τ, η, η′) I m (τ, η, η′)d η′ + 2  τ  Λ (τ) S p m (τ, η, η′) exp  −  + 4  η0 

B m (τ, η, η0 ) =

(8.2.15)

Finally, for the boundary conditions (8.1.10) we get I m (0,η,η 0 ) = 0, if η>0, I m (τ 0 ,η,η 0 ) = 0, if η 0 η0 η   τ

∫

 τ − τ′  1 I ( τ, η, η0 ) = − B (τ′, η, η0 ) exp  −  d τ′, if η < 0, ητ η   τ0

m

(8.2.17)

whose substitution into (8.2.15) gives the integral equation for the function of sources

B m (τ, η, η0 ) =

∫

∫

d η′ Λ( τ)  m  p ( τ, η, η′) ′ B( τ′, η′, η0 ) × 2  −1 η 0 τ

0

∫

 τ − τ′  d η′ d τ′ − p m (τ, η, η′) × exp  − ×  η′  η′  −1 0

∫

 τ − τ′   × B m (τ′, η′, η0 )exp  −  d τ′  +  η′   τ τ0

+

(8.2.18)

 τ  Λ(τ) m Sp (τ, η, η0 )exp  −  . 4  η0 

Expansions (8.2.12) and (8.2.13) are often referred to as expansions by azimuthal harmonics and the functions I m(τ,η,η 0) and B m (τ,η,η 0 ) – as azimuthal harmonics. Usually in the transfer theory it is preferred to operate with the azimuthal harmonics and the equations for them.

Coefficients of reflection and transmission of the atmosphere In many problems it is not necessary to calculate the intensity of scattered light in the thickness of the atmosphere, i.e. in relation to τ, and it is sufficient to know the intensity of the radiation outgoing from the atmosphere. For example, problems of this type 349

Theoretical Fundamentals of Atmospheric Optics

appear in interpreting the measurements of the intensity of scattered radiation from satellites and the brightness of sky from the surface of the Earth. In this case, the required intensity can be conveniently represented in the form [47, 65] I(0,–η,η 0 ,ϕ) = Sη 0 ρ(η,η 0 ), I(τ 0 ,η,η 0 ,ϕ) = Sη 0 σ(η,η 0 ).

(8.2.19)

Quantities ρ(η,η 0 ,ϕ) and σ(η,η 0 ,ϕ) are referred to respectively as the coefficients of reflection and transmission of the atmosphere. Since F = S 0 is the flux incident on the area parallel to the surface on the upper boundary of the atmosphere, then ρ(η,η 0 ,ϕ) = πI(0,–η,η 0,ϕ)/F, but this definition in accuracy coincides with the definition (6.3.21) of the coefficient of spectrum brightness. Expanding ρ(η,η 0,ϕ) and σ(η,η 0 ,ϕ) in respect of the azimuthal harmonics ρ(η, η0 , ϕ) = ρ0 (η, η0 ) + 2

∑ ρ (η, η )cos mϕ,

σ(η, η0 , ϕ) = σ (η, η0 ) + 2 0

N

m

0

m=1

∑ σ (η, η )cos mϕ,

(8.2.20)

N

m

m =1

0

we obtain I m (0,–η,η 0 ) = Sη 0 ρ m )(η,η 0 ), I m (τ 0 ,–η,η 0 ) = Sη 0 σ m )(η,η 0 ).

(8.2.21)

Reflection of scattered radiation from the orthotropic surface Previously, we presented equations for the transfer of exclusively scattered radiation. We now take into account the reflection of light from the underlying surface. To simplify considerations, we examine an orthotropic surface with the albedo equal to A. In this case, in solving the transfer equation, the presence of the reflecting surface influences only the zero azimuthal harmonics. To confirm this claim, it is sufficient to use the ‘proof by contradiction’ principle: actually, if the orthotropic surface affected the non-zero harmonics of intensity then in accordance with (8.2.12) it would also affect its dependence on the azimuth (through cos mϕ), but this contradicts the fact that the surface is orthotropic, i.e. reflecting in the same manner on all azimuths. Taking this claim into account, we omit the indexes zero at the 350

Main Concepts of the Theory of Solar Radiation Transfer

coefficients of reflection and transmission. In ‘addition’ reflection from the underlying surface, coefficients of reflection and transmission of the atmosphere change. We introduce the following notations [65]. As previously, the coefficients of reflection and transmission without taking the surface into account are denoted by ρ(η,η 0 ) and σ(η,η 0 ). Identical coefficients, but already with the – reflection from the surface taken into account, are denoted as ρ – (η,η 0 ) and σ(η,η 0 ). We also need the coefficients of reflection and transmission in illuminating the atmosphere from below in the absence of the surface, i.e. from the point τ 0 , and we denote them as ρ∼ (η,η 0 ) and σ∼ (η 0 ,η). The appropriate designations are also introduced for intensity. It should be mentioned that in a general case, the symmetric relationships hold for the coefficients of reflection and transmission of the atmosphere: ρ(η,η 0 ,ϕ) = ρ(η 0 ,η,ϕ),

(8.2.22)

ρ∼ (η,η 0 ,ϕ) = ρ∼ (η 0 ,η,ϕ), σ(η,η 0 ,ϕ) = σ∼ (η 0 ,η,ϕ). We accept that without proof which requires a relatively detailed analysis of the integral equation for the function of sources (8.2.18) [65] which is outside the examined subject of taking into account reflection from the surface. We only note that in Chapter 6 we proved the symmetry of the coefficient of spectral brightness of the surface (6.3.15). This principle is also maintained in complex processes of multiple light scattering. We determine the flux incident from the atmosphere on the surface

∫ ∫

2π

1

0

0

F ↓ (η0 , τ0 ) = d ϕ I (τ0 , η′, η0 )η′d η′ +

 τ  +πS η0 exp  − 0  .  η0 

(8.2.23)

The first term in (8.2.23) is a hemispherical flux of scattered radiation, according to (3.2.9), the second one is the contribution of direct solar radiation according to the Bouguer law. Direct radiation must be added because we have agreed to examine the

351

Theoretical Fundamentals of Atmospheric Optics

equation of transfer and, consequently, determine the intensity and the coefficients of reflection and transmission (according to (8.2.19)) only for scattered radiation. Since according to the definition of the surface albedo (6.3.11) the upward flux F ↑ (η 0 ,τ 0 ) is F ↑ (η 0,τ 0) = AF ↓ (η 0,τ 0), then expressing the intensities through the transmission coefficient according to (8.2.21) we obtain

∫

1   τ  F ↑ (η0 , τ0 ) = A  2πS η0 σ(η′, η0 )η′d η′ + πS η0 exp  − 0   . (8.2.24) 0  η0   

The presence of the surface is equivalent to elimination of the atmosphere from below. The calculation of the intensity generated by this illumination at the boundaries of the atmosphere is complicated by the fact that in contrast to illumination from above the light from the bottom comes from different directions. To reduce this case to the already examined case, initially we examine illumination from below only and the one angle with the cosine η' > 0 (Fig. 8.2), and here η' > 0 because we deal with the ‘inverted’ geometry (the pattern should be equivalent to that illuminated from above). We can introduce, at the moment formally, a flux incident on the area normal to the rays and equal to, as in ~ the case of the flux from below, πS (η'). Now, according to (8.2.19), the intensity of scattered radiation at the upper boundary ~ is S (η') σ η'(η,η'). However, the atmosphere is illuminated from the bottom not only from one direction η' and, therefore, to determine intensities at the lower and upper boundaries, this expression should be integrated in all directions. In addition to this, for the upper boundary it is important to take into account the direct radiation, coming from the surface, in exactly the same manner as in the case of illumination from above. Consequently, we can write

∫ ∫

 τ  I (0, η) = d ϕ S (η′)η′σ (η, η′) d η′ + πS (η) exp  − 0 ,  η 0 0 2π

1

∫ ∫

2π

1

0

0

I (τ0 , η) = d ϕ S (η′)η′ρ(η, η′)d η′.

It is now easy to determine the intensity at the boundaries of the atmosphere in the presence of the surface. Actually, the intensity at the upper boundary is the sum of intensity resulting only from scattering in the atmosphere I(0,η,η 0 ) and the intensity as a result 352

Main Concepts of the Theory of Solar Radiation Transfer

Fig. 8.2. Illumination of the atmosphere from the bottom.

of the illumination of the atmosphere by the light reflected from the ~ surface I (0,η). Similarly, the intensity at the lower boundary is the sum of the intensity of the light scattered in the atmosphere I(τ 0 ,η,η 0 ) and the intensity as a result of illumination by the ~ reflected light I (τ 0 ,η). Thus,

∫

I (0, η, η0 ) = Sη0 ρ(η, η0 ) + 2 π S (η′)η′σ(η, η′)d η′ + 1

0

 τ  +πS (η) exp  − 0  ,  η

(8.2.25)

∫

I (τ0 , η, η0 ) = S η0 σ(η, η0 ) + 2π S (η′)η′ρ(η, η′)d η′. 1

0

~ We now determine S (η'). From the fact that the flux on the area, normal to the rays, is numerically equal to the intensity (Chapter ~ 3), it is concluded that πS (η') = I r(η'), where I r (η') is the intensity of the light reflected from the surface in the direction η'. However, for the orthotropic surface, the reflected intensity does not depend on the direction (Chapter 6), i.e. I r (η') ≡ I r , i.e. I r is a constant. Consequently

∫ ∫

2π

0

d ϕ I r η′d η′ = F ↑ (η0 , τ0 ), 1

0

from which 353

Theoretical Fundamentals of Atmospheric Optics

I r = F ↑ (η0 , τ0 ) π and S (η′) = I r π = F ↑ (η0 , τ0 ) π 2 .

Substituting the last equation into (8.2.25) we get

I (0, η, η0 ) = Sη0 ρ(η, η0 ) +

+

∫

1 F ↑ (η0 , τ0 )   τ0    2 σ (η, η′)η′d η′ + exp  −   , π  η   0

I (τ0 , η, η0 ) = S η0 σ(η, η0 ) +

∫

F ↑ (η0 , τ0 ) 2 ρ(η, η′)η′d η′. π 0 1

Dividing both parts of the equalities by Sη 0 , according to the definitions (8.2.19), we obtain ρ(η, η0 ) = ρ(η, η0 ) +

∫

 1  τ  +β(η0 , τ0 )  2 σ ( η, η′ ) η′d η′ + exp  − 0   ,  η   0

(8.2.26)

∫

σ(η, η0 ) = σ(η, η0 ) + 2β(η0 , τ0 ) ρ(η, η′)η′d η′, 1

0

where β(η0 , τ0 ) = F ↑ (η0 , τ0 ) (πS η0 ) and according to (8.2.24):

∫

 1  τ  β(η0 , τ0 ) = A  2 σ(η′, η0 )η′d η′ + exp  − 0   .    η0    0

(8.2.27)

We can now express the coefficients of reflection and transmission of the atmosphere with the accounting surface through identical coefficients for the case with the surface not taken into account. Substituting (8.2.26) into (8.2.27) we get

∫

 1 β(η0 , τ0 ) = A  2 σ(η′, η 0 )η′d η′ +  0

∫

∫

1  τ  ′ ′ +4β(η0 , τ0 ) η d η ρ(η′, η′′)η′′d η′′ + exp  − 0   . 0 0  η0   1

354

Main Concepts of the Theory of Solar Radiation Transfer

Consequently

∫

 1  τ  A  2 σ(η′, η0 )η′d η′ + exp  − 0   0  η0   . β(η0 , τ0 ) =  1 1 ′ ′ ′ 1 − 4 A ηd η ρ(η, η )η d η

∫ 0

∫ 0

To shorten the equations we introduce the notations [65]

∫

∫

C = 4 ηd η ρ(η, η′)η′d η′, 1

1

0

0

∫

E (η) = 2 ρ(η, η′)η′dη′, 1

0

∫

 τ  V (η0 , τ0 ) = 2 σ(η, η0 )ηd η + exp  − 0  , 0  η0  1

(8.2.28)

∫

1  τ  V (η, τ0 ) = 2 σ(η, η′)η′ d η + exp  − 0  = 0  η0 

∫

1  τ  = 2 σ(η′, η)η′ d η′ + exp  − 0  = V (η, τ0 ), 0  η0  ~ where in deriving the equality V (η,τ 0 ) = V(η,τ 0 ) we took into account the symmetry ratio (8.2.22). The equations (8.2.26) and (8.2.27) now give

ρ(η, η0 , τ0 ) = ρ(η, η0 , τ0 ) +

AV (η, τ0 )V (η0 , τ0 ) , 1 − AC

AE (η)V (η0 , τ 0 ) σ (η, η0 , τ0 ) = σ(η, η0 , τ 0 ) + . 1 − AC

(8.2.29)

It should be mentioned that for the coefficient of reflection of the atmosphere the symmetry property is also maintained when reflection from the surface is taken into account – – ρ (η,η 0 ,τ 0 ) = ρ (η 0 ,η,τ 0 ). 355

Theoretical Fundamentals of Atmospheric Optics

Thus, solving the equation of transfer and finding the coefficients of reflection and transmission without considering the presence of the surface, from (8.2.28) and (8.2.29) we can easily determine them already taking into account the orthotropic surface. Of course, it is now necessary to solve the equation for the ‘inverted’ atmosphere in order to find ρ (η,η 0), but only for zero harmonic and the equation of transfer is the simplest. In a general case of calculating the intensity inside the atmosphere and also calculating the intensity in reflection from a non-orthotropic surface, using the same method of summation of the intensities in illumation from top and bottom, it is also possible to express the parameters of the ‘atmosphere plus surface’ system through the parameters of the atmosphere ‘without the surface’ [65]. Thus, in the modern theory of transfer, the problem of reflection from the surface is solved analytically and we can examine the transfer of exclusively scattered radiation without taking direct and reflected radiation into account.

8.3. Numerical methods in the theory of radiation transfer Specific features of numerical methods A practical problem of the theory of transfer radiation is the development of numerical algorithms of computer calculations of the intensity of radiation with multiple scattering taken into account. The importance of the problem results from the need for interpreting the results of measurements of the intensity of scattered light and also calculating the hemispherical fluxes of solar radiation which determine the energy regime of the atmosphere. It should be mentioned that the complexity and specific features of the transfer equation create problems in numerically solving this equation by standard computing methods (substitution of derivatives by finite differences, substitution of integrals by finite sums). Therefore, special algorithms have been developed for this equation and the concepts used as a basis for these algorithms greatly differ. At present, there are more than ten methods of numerical solution of the transfer equation which greatly differ in their ideology [53]. We select only four of them illustrating this difference of concepts using these methods as an example. It should be mentioned that because of the previously mentioned special features of the transfer equation, the development of numerical methods of solving this equation is closely connected with the analytical methods and it is often difficult to separate these two approaches, as will be shown 356

Main Concepts of the Theory of Solar Radiation Transfer

on examples of the methods of spherical harmonics and composition of the layers.

The method of spherical harmonics This method is used for determining the intensities from the integrodifferential equation (8.1.9), but we shall clarify its concept using a simpler integral equation for the function of sources (8.2.18) [65]. We find the unknown functions B m (τ,η,η 0 ) in the form of an expansion into a series in terms of the Legendre adjoint functions identical with (8.2.11), separating thus the variables η and a τ: B m (τ, η, η0 ) =

∑c N

i =m

m

i

(τ) Pi m (η) Bim (τ, η0 ),

(8.3.1)

(i − m)! . The functions to be determined are (i + m)! ~m Bi (τ,η 0). Substituting (8.2.11) and (8.3.1) into the integral equation ~ (8.2.18) and equating the coefficients at Bim (τ,η 0 ) with equal m and i, we obtain

where cim (τ) = xi (τ)

Bim (τ, η0 ) =

Λ(τ) 2

∑ N

i =m

∫

1 d η′ × cim (τ)  Pi m (η′) Pi m (η′) η′ 0

∫

ι  τ − τ′  × Bim (τ, η0 ) exp  −  d τ′ −  η′  0

∫

− Pi m (η′) Pi m (η′) 0

−1

+

∫

ι0   τ − τ′  d η′ m B j ( τ, η0 ) exp  − d τ′  +  η′ ι  η′  

(8.3.2)

 τ Λ(τ) m SPi (η0 )exp  −  , 4  η0 

where m = 0, …, N; i = m, …, N. The relationships (8.3.2) are a system of integral equations for ~ determining the unknown functions Bim (τ,η 0 ). It would appear that we have not gained anything by obtaining, instead of N independent equations (8.2.18), a system of

1 ( N + 1)( N + 2 ) equations. However, 2

357

Theoretical Fundamentals of Atmospheric Optics

firstly, the system (8.3.2) as may easily be seen is ‘triangular’: the equation with m = N includes only one unknown function

( B ( τ,η ) ) , the ( B ( τ,η ) and B N N

N −1 N

0

0

N N

)

equation with m = N–1, i = N only two (τ, η0 ) and so on. Thus, the system should be

BNN ( τ ,η0 ) , then BNN −1 ( τ ,η0 )

solved by the ‘method of inverse course’: initially, to determine and so on. Secondly, exchanging

integrations in respect of η' and τ' and taking into account the explicit expressions for the Legendre adjoint functions according to (8.2.9), integration in respect of η' may be carried out analytically. Its result is expressed in the form of a series of the already known special functions – integral exponents E n (x) (see paragraph 7.5) of the argument (τ,−τ'). In a general case, the derivation of formulas for the coefficients of the series and the results are very combersome and we shall therefore not discuss this and only confirm the possibility of retaining integration only in respect of τ in equations (8.3.2). Therefore, thirdly, the obtained integrals are one-dimensional and it is now easy to solve them by the standard numerical methods of solving integral Fredholm equations of the second kind: reduction to the system of algebraic linear equations by representing the function at discrete values τ or the iteration methods, based on expansion into a Neumann series (8.1.17).

The method of discrete ordinates This method is close to the ‘standard’ schemes of numerical solution of differential equations. It is based on replacement in the integrodifferential equation (8.2.18) of the integral in respect of the angles by the Gauss quadrature equation, i.e. transition to the discrete grid for the scattering angles (thus the name of the method). Consequently, equation (8.2.18) transfer to a system of ordinary differential equations of the first order. The method of solving these systems are well known and based on the determination of the eigennumbers and vectors of their matrices. In the case of the transfer equation, this matrix may be converted to a special form greatly simplifying the solution procedure [37].

The method of addition of layers We examine a partial problem of determination of the intensity of radiation at the boundaries of the atmosphere. The method of 358

Main Concepts of the Theory of Solar Radiation Transfer

addition of layers is based on the method of addition of intensities if they are known for every layer separately, in combining two atmospheric layers. Of course, this is the simplest and at the same time sufficiently effective method of calculating intensity [53]. If we add to the layer with multiple scattering a very thin layer whose parameters may be regarded as constant in respect of τ and the scattering in this layer is only single, the expressions for the intensities at the boundaries of the combined layer are obtained in the explicit form. Dividing the entire atmosphere into such thin layers, we can added them successfully. To describe the method, we use the equations of intensities for the layer with single scattering illuminated from different directions. All considerations will be made for the expansion of intensities in respect of the azimuthal harmonics. In the case of single scattering of light and constant parameters of the layer, the function of sources (8.2.15) has the form

B m (τ, η, η0 ) =

Λ m Sp (η, η0 )exp(− τ η0 ). 4

Substituting this equation into the expression for intensity through the function of sources (8.2.17) at the boundaries of the layer (τ = 0 and τ = ∆τ) and to stress the approximation of single scattering, denoting the intensities by letter i, we obtain i m (0, −η, η0 ) = =

∫

1Λ m Sp (η, η0 ) exp(−τ′ η0 ) exp(−τ′ / η) d τ′ = η4 0 ∆ι

1Λ m 1 − exp(−∆τ(1 η0 + 1 η)) Sp (η, η0 ) , η4 1 η0 + 1 η

i m (∆τ, η, η0 ) =

∫

1Λ m Sp (η, η0 ) exp(−τ′ η0 ) × η4 0 ∆τ

 ∆τ − τ′  × exp  −  d τ′ = η  

=

1Λ m 1 − exp(−∆τ(1 η0 − 1 η)) . Sp (η, η0 ) exp(−∆τ η) η4 1 η0 − 1 η

359

Theoretical Fundamentals of Atmospheric Optics

Finally,

i m (0, −η, η 0 ) =

Λ 1 − exp( −∆τ (1/ η 0 + 1/ η) S η 0 p m ( η, η 0 ) , η + η0 4 i m ( ∆τ, η, η0 ) =

=

(8.3.3)

Λ exp(−∆τ η) − exp(−∆τ η0 ) . S η0 p m (η, η0 ) η − η0 4

Evidently, because of the constancy of the parameters of the layers if the layer is illuminated from below i m (0, −η, η0 ) = i m (0, −η, η0 ) m and i (∆τ, η, η0 ) =i m (∆τ,η,η 0 ). We now examine a layer not illuminated with direct rays but illuminated with radiation with the intensity Im0 (η), and assuming that the coefficient of reflection and transmission of the layer ρ m(0,η,η 0 ) and σ m (∆τ,η,η 0 ) are given, we determine the intensities on the upper and lower boundaries of the layer I m (0,–η), I m (∆τ,η). It should be noted that, generally speaking, the scattering in the layer is multiple. This problem has already been solved in the previous paragraph but the results were not given in the explicit form. We examine only one direction of the radiation η' incident on the layer and introduce formally for it the intensity of the incident flux πS m(η'). Then, according to (8.2.21) we have I m (0,–η,η') = S m (η')η'ρ m (η,η'), I m (∆τ,η,η') = S m (η')η'σ m (η,η'). To determine the total intensity we integrate with respec to all directions of incidence and for transmission we take into account direct radiation

∫ ∫

2π

1

0

0

I m (0, −η) = d ϕ S m (η′)η′ρ m (η, η′)d η′,

∫ ∫

2π

1

0

0

I (∆τ, −η) = d ϕ S m (η′)η′σ m (η, η′)d η′ + m

(8.3.4)

+πS (η)exp( −∆ι η).

Further, taking into account that πS(η',ϕ) = I 0 (η',ϕ), and expanding both parts in respect of the azimuthal harmonics, we obtain 360

Main Concepts of the Theory of Solar Radiation Transfer

S m (η′) =

1 m I 0 (η′) . Substitution of this expression into (8.3.4) gives the π required result:

∫

I m (0, −η) = 2 I 0m (η′)η′ρ m (η, η′)d η′, 1

0

∫

I m (∆τ, η) = 2 I 0m (η′)η′σ m (η, η′)d η′ + 1

(8.3.5)

0

+ I 0m (η) exp(−∆τ η).

For the method of composition of layers it is however convenient to write (8.3.5) not through the coefficients of reflection and transmission but through the intensity of radiation emerged from the layer illuminated by direct rays with the cosine of the angle η'. From (8.2.21) we obtain ρm (η, η′) =

1 m I (0, −η, η′), Sη′

σ m (η, η′) =

1 m I (0, −η, η′), Sη′

and therefore

I m (0, −η) =

2 m I 0 (η′) I m (0, −η, η′)d η′, S

I m ( ∆τ, η) = I 0m (η) exp( −∆τ η) +

+

(8.3.6)

∫

2 m I 0 (η′) I m (∆τ, η, η′) d η′. S0 1

It should be mentioned that the left part in (8.3.6) contains the intensitie for the case of lightning by radiation I m0(η') and under the integrals – in illumination by direct rays from the direction η'. For the latter we haven’t introduced any special notation, and they will be distinguished on the basis of the number of arguments. In the case of lighting the layer from below all considerations remain the same. However, in order to remain in the system of 361

Theoretical Fundamentals of Atmospheric Optics

co-ordinates similar to the case of direct radiation, it is necessary to ‘invert’ the co-ordinate axes (to exchange ∆τ and 0) and all ~ intensities should be denoted by I .

∫

2 m I (∆τ, η) = I 0 (η′) I m (∆τ, η, η′)d η′, S 0 1

m

I m (0, −η) = I 0m (η)exp(−∆τ η) + +

(8.3.7)

∫

2 m I 0 (η′)I m (0, η, η′)d η′. S0 1

We now pass directly to the method of composition of layers (Fig. 8.3). Let us assume that the first layer has the optical thickness τ and is illuminated by direct solar radiation with the cosine of zenith angle η 0 . Assume that we have obtained (taking into account multiple scattering) the intensities outgoing from the layer for the case of illuminating it from above and below ~ ~ I m1 (0,–η,η 0 ), I m1 (τ,η,η 0 ), I m1 (0,–η,η 0 ) and I 1m (τ,η,η 0 ). To this we add from the bottom a thin layer with a thickness ∆τ in which scattering is assumed to be single, the layer parameters are assumed to be independent of τ and, consequently, the intensities leaving this layer are determined by equations (8.3.3). It is required to find the intensities of radiation outgoing from the combined layer: ~ ~ I m (0,–η,η 0 ), I m (τ+∆τ,η,η 0 ), I m(0,–η,η 0 ) and I m(τ+,∆τ,η,η 0 ). The key to solving the problem is the determination of the intensities at the contact boundaries of the layers. We determine I m (τ,–η,η 0 ) – the intensity of radiation from the first into second layer. The second layer is now illuminated from above by direct radiation which, however, is weakened in accordance with Bouguer’s law during passage through the first layer, i.e. instead of S for the second layer we should now take S exp(–τ/η 0 ). In addition to this, the second layer is illuminated from above by the radiation from the first layer I m0 (η) = I m1 (τ,η,η 0 ). Generally speaking, this intensity should change because of the scattering in the first layer of radiation incoming from the second layer. However, if this change is taken into account, the scattering in the second layer will no longer be single scattering (we take into account the scattering of radiation whose change has already been caused by the scattering in the second layer). Thus, remaining in the framework of approximation of single scattering in the second layer, for the intensity I m (τ,–η,η 0 ) we obtain not an equation but 362

Main Concepts of the Theory of Solar Radiation Transfer

Fig. 8.3. Method of composition of layers.

an explicit relationship

I m ( τ, −η, η0 ) = i m (0, −η, η0 ) exp(−τ η0 ) +

+

∫

2 m I1 (τ, η′, η0 )i m (0, −η, η′)d η′. S 0 1

(8.3.8)

The first term is the intensity of direct radiation reflected by the second layer, taking into account the extinction of the incident direct radiation by the first layer, the second term is the reflected radiation according to (8.3.6). ~ We find I m(τ,–η,η 0 ) – the intensity of radiation passed through the second layer in the case of illuminating from below. The second layer is now illuminated from above below by direct radiation incident directly on its boundary (without extinction), and from above ~ by the radiation of the first layer with the intensity I m0 (η) = ~m I 1 (τ,η,η 0 ). The required intensity is the sum of the radiations transmitted in the case of lighting from below and reflected in illumination from above:

I m (τ, −η, η0 ) = i m ( ∆τ, −η, η0 ) +

∫

2 m + I1 (τ, η′, η0 )i m (0, −η, η′)d η′. S 0 1

363

(8.3.9)

Theoretical Fundamentals of Atmospheric Optics

At the boundary of the layers we now determine the intensities of radiation outgoing from the first layer. To determine I m(τ,η,η 0 ) we take into account the illumination of the layer from above, giving I m1(τ,η,η 0 ), it is necessary to add the illumination from below with the intensity determined previously for which the reflection should be taken into account in accordance with (8.3.7). Consequently I m ( τ, η, η0 ) = I1m ( τ, η, η0 ) +

∫

2 m + I (τ, – η, η0 ) I1m (τ, η, η′)d η′. S 0 1

(8.3.10)

~ To determine I m (τ,η,η 0 ) it is necessary to take into account the direct radiation incident on the first layer from the bottom (however, this radiation was attenuated during the passage through the second layer by exp(–∆τ/η 0 ) times, and the illumination of the first layer ~ from the same side with intensity I m (τ,–η,η 0 ). In both cases, we have a reflection. Consequently

I m (τ, η, η0 ) = I1m (τ, η, η0 )exp(−∆τ η0 ) + +

∫

2 m I (τ, – η, η0 ) I1m (τ, η, η′)d η′. S 0 1

(8.3.11)

Using the previously determined intensities on the level of the contact of the layers we find the required intensities at the boundaries of the already combined layer. Here there are again four cases. For I m ( τ+∆τ,η,η 0 ) we must take into account the transmission, by the second layer, of the incident direct radiation attenuated by the first layer plus the transmission of the radiation with I m0 (η) = I m (τ,η,η 0 ). We obtain I m ( τ + ∆τ, η, η0 ) = i m (∆τ, η, η0 ) exp(−τ η0 ) + + I m ( τ, η, η0 ) exp( −∆τ η) +

+

(8.3.12)

∫

2 m I (τ, η′, η0 )i m (∆τ, η, η′)d η′. S 0 1

~ For I m(τ+∆τ,η,η 0 ) we have the direct radiation incident directly 364

Main Concepts of the Theory of Solar Radiation Transfer

on the second layer. For this radiation we taken into account the reflection to which it is necessary to add the transmission from radiation with I m0 (η) = I m (τ,η,η 0 ):

I m (τ + ∆τ, η, η0 ) = i m (0, −η, η0 ) + + I m (τ, η, η0 )exp(−∆τ η) +

(8.3.13)

∫

2 m I (τ, η′, η0 )i m ( ∆τ, η, η′)d η′. + S 0 1

For I m (0, –η,η 0 ) when the layer is illuminated from above and ~ transmission in the case of illuminating from below with I 0m (η) = ~m I (τ,–η,η 0 ): I m (0, – η, η0 ) = I1m (0, −η, η0 ) +

+ I m (τ, – η, η0 )exp(−τ η) +

+

(8.3.14)

∫

2 m I (τ, – η′, η0 ) I1m (0, η, η′)d η′. S 0 1

~ Finally, for I m (0,–η,η 0 ) we have illumination from below, attenuated by the second layer, and the illumination also from below ~ with I 0m =I m(τ,–η,η 0 ):

I m (0, – η, η0 ) = I1m (0, −η, η0 )exp(∆τ η0 ) + + I m (τ, – η, η0 )exp(−τ η) + +

(8.3.15)

∫

2 m I (τ, – η′, η0 ) I1m (0, η, η′)d η′. S 0 1

Equations (8.3.3), (8.3.8)–(8.3.15) include the algorithm of the method of composition of the layers. In fact, using the thin layer of the atmosphere as the first layer and finding the intensities of this layer at its boundaries in the approximation of single scattering (8.3.3) we can gradually add thin layers to this layer until the entire atmosphere is exhausted. It should be taken into account that the method gives accurate results only for the boundaries; the determined intensities inside the atmosphere are of auxiliary nature and cannot be used for determining the vertical dependence of 365

Theoretical Fundamentals of Atmospheric Optics

intensity. In fact, for this purpose it would be necessary to recalculate all intermediate intensities when adding every new layer and this is not done (and cannot be done in the frames of this method). It should also be mentioned that thin layers with single scattering are usually represented by layers with ∆τ of the order of 0.01–0.05.

Monte Carlo method* This is one of the most powerful computing methods of the transfer theory which makes it possible to solve numerically the problems which are ‘not solvable’ by other methods [46]. However, we examine the simplest realisation of the Monte Carlo method with an example of calculating hemispheric fluxes. Finally, it is possible to calculate flows after calculating intensities, integrating them in respect of angles, but in transfer theory there are also simpler methods which make it possible to calculate hemispherical flows directly. One of these is the Monte Carlo method. The concept of the Monte Carlo method is the representation of radiation transfer in the atmosphere in the form of a random process and modelling of this process in a computer. For statistical computer-based modelling it is necessary to have a device playing the role of ‘a blind case’. Similar algorithms, referred to as ‘random number generators’ are well known at the present time and we shall not discuss them. The application of the Monte Carlo method requires a generator of uniformly distributed random numbers in a range [0, 1]. We agree to denote these random numbers by α and when it appears in the text it will denote a new random number. In the Monte Carlo method radiation transfer is regarded as movement through a medium of photons. To clarify the concept of the method we discuss a simpler case. Let it be that an atmosphere whose optical thickness τ 0 is illuminated by the Sun at the angle with cosine η 0 by the flux πS on an area normal to the rays. From Bouguer’s law we know that the extinction of direct radiation in passage through such an atmosphere is equal to P = exp(–τ 0 /η 0 ). However, quantity P can also be interpreted as the probability p of the photon passing through the atmosphere without any interaction with it. We examine gradually the movement of the photons through the atmosphere. For each photon we use a random number of a = The other, more accurate term is the method of statistical modelling but the ‘Monte Carlo method’ is preferred. The name comes from the name of the Mediterranean resort. 366

Main Concepts of the Theory of Solar Radiation Transfer

α (every time a new number) and if a < p the photon has passed through the atmosphere without interaction (if a > p then not). We count the number of the transmitted photons: let it be that their number is N(τ 0) and the total number of photons is N. Because of the uniformity of the generator of random numbers it may be asserted that at high N the fraction of transmitted photons is equal to p but this means that N(τ 0 ) = N exp(–τ 0 /η 0 ), i.e. N(τ 0 ) is the flux of direct (not interacting with the atmosphere) radiation at the lower boundary, but only in the units of the number of photons N. To reduce to energy units it is necessary to multiply it by the expression of the flux at the upper boundary for a single photon, i.e. by πSη 0 /N. Thus, for direct radiation we have

F ↓ ( τ0 ) =

N ( τ0 ) πS η0 . N

(8.3.16)

Finally, the result (8.3.16) was known previously. We used it to explain that, simulating the transfer of radiation through the atmosphere as a random process and counting the photons, we may obtain the required values of fluxes if the number of photons is large. To extend this result to the case of multiple scattering it is necessary to model all three process: free path of the photon, its interaction with the atmosphere (absorption and scattering) and its interaction with the surface (absorption and reflection). The free path of the photon is its displacement without interaction with the atmosphere. We have already mentioned this. Let it be that the photon is on the level τ 1 and moves at the angle with cosine η. The probability of the passage of the path ∆τ is exp(–∆τ/|η|). However, this relationship can also be interpreted differently: we use the random number a = α, which will be regarded as the given probability and from this number we determine ∆τ: exp(–∆τ/|η|) = a; ∆τ = –ηlna. It is precisely a random model of the free path. Now, the new position of the photon in the atmosphere τ 2 is determined from the equation τ 2 = τ 1 –τln a

(8.3.17)

and taking into account that ln a < 0, equation (8.3.17) also holds in motion downwards (η > 0) and upwards (η< 0). If in modelling using (8.3.17) τ 2 < 0, the photon has left the atmosphere and it is necessary to ‘launch’ the next one, if τ 2 > τ 0 , the photon has reached the surface and it is necessary to model interaction of the photon with the surface; otherwise (0 < τ 2 < τ 0 ) – photon has 367

Theoretical Fundamentals of Atmospheric Optics

remained in the atmosphere and it is necessary to model its absorption or scattering in the atmosphere. We now examine the interaction of the photon with an orthotropic surface. According to the definition of the albedo, it is the fraction of reflected radiation. Transferring this claim to the language of probabilities, identical with that made previously for the fraction of direct radiation, we immediately obtain that the albedo A is the probability of reflection of the photon from the surface. Then, if α < A, the photon has been reflected, otherwise it is absorbed by the surface and it is necessary to model a new photon. In the reflection process a photon obtains a new direction arccos η 2 and ϕ 2 . Since the surface is orthotropic, they are all of equal probability and are independent of the direction of incidence on the surface η 1 and ϕ 1 . However, in this case they can be modelled quite simply using the uniform distribution

π  η2 = − cos  α  , ϕ2 = 2πα, 2 

where the letters α denote the different random numbers. The case of interaction with the atmosphere is slightly more complicated. Let us assume that this interaction takes place on level τ 1 and the direction of the photon is (η 1 , ϕ 1 ). The meaning of the albedo of single scattering Λ(τ 1 ) as already shown in explaining this term in paragraph 8.1 is completely identical with the meaning of the albedo of the surface: Λ(τ 1 ) is the probability of scattering of the photon. It appears that if α < Λ(τ 1 ) scattering takes place, otherwise the photon is absorbed in the atmosphere and it is necessary to start modelling the trajectory of the next photon. In scattering it is necessary to determine the angle and azimuth of scattering. We should mention the probability interpretation of the scattering phase function (see Chapter 3): the phase function x(γ) is the probability density of light scattering at the angle γ. According to the definition of the probability density, the probability of scattering in the angle range from 0 to γ is p=

∫ γ

1 x( γ )sin γ d γ, 20

where 1/2 is taken from the normalisation condition (3.3.13). We ‘invert’ this relationship using as the probability the random number a = α and pass from the angles to cosines (see paragraph 8.1). We obtain an equation for determining the cosine of the angle of 368

Main Concepts of the Theory of Solar Radiation Transfer

scattering η in random modelling

∫ x(τ , ω)d ω = 2α. η

(8.3.18)

1

−1

Usually, in the Monte Carlo method the phase function is given in the form of a table in respect of argument ω: x(ω i ), i = 1, …, M, ω 1 = –1, ω M = 1. Consequently, writing integral (8.3.18) on this grid through the quadrature trapezidal formula we easily obtain an explicit expression for η. Actually, we examine a table of quantities

∫

ωi

Si = x(τ1 , ω′) d ω′ = −1

∑ 2 ( x(τ , ω i −1

1

j =1

1

i +1

) + x(τ1 , ω j ))(ω j +1 − ω j ).

We determine number k from the condition S k ≤ 2a ≤ S k+1 . Evidently, the required point η is between ω k and ω k+1 and

∫ η

1 2a = S k + x(τ1 , ω′) d ω′ = S k + ( x(τ1 , η) + x(τ1 , ωk ))(η − ωk ). 2 ω k

In the trapezoid rule, the subintegrand is approximated by a linear function and, consequently, using linear interpolation, we have

x(τ1 , η) = x(τ1 , ωk ) +

x(τ1 , ωk +1 ) − x(τ1 , ωk ) (η − ωk ) ωk +1 − ωk

and to determine η we obtain the quadratic equation d k (η – ω k ) 2 + 2x k (η – ω k ) + (2S k – 4a) = 0, with the notations

dk =

x(τ1 , ωk +1 ) − x(τ1 , ωk ) , xk = x(τ1 , ωk ). ωk +1 − ωk

The solution of the equation is: η = ωk +

− xk + xk2 + dk (4 a − 2 Sk ) dk

,

(8.3.19)

with the plus sign in front of the root selected from the condition ω k < η ≤ ω k+1 . As regards the azimuth of scattering ϕ, since the phase function is independent of the azimuth, as in the case of reflection from the surface, then 369

Theoretical Fundamentals of Atmospheric Optics

ϕ 2 = 2πα.

(8.3.20)

After modelling the angle and azimuth of scattering by (8.3.19), (8.3.20), it is necessary to find the new direction of the photon (η 2, ϕ 2 ). It is determined from the well known equations of spherical trigonometry [67]:

η2 = η 1η − (1 − η12 )(1 − η2 ) cos ϕ,   η − η1η2 ϕ2 = ϕ1 + arccos  .  (1 − η2 )(1 − η2 )  1 2  

(8.3.21)

(8.3.22)

Thus, we have learnt how to model all processes of interaction of the photon in the atmosphere. Now, we gradually simulate N trajectories of photons (usually for fluxes we use N of the order of thousands). At the beginning of the trajectory, every photon has the co-ordinate τ = 0, η = η 0 , ϕ = 0. We simulate its free path and an interaction with the atmosphere or the surface, and then a new free path, and so on, until the photon emerges through the upper boundary of the atmosphere or is absorbed by the atmosphere on the surface. To calculate the fluxes on the level τ, we count the number of photons passing through the given level, i.e. the number of cases when τ 2 < τ < τ 1 at η > 0 for F ↓ (τ) and when τ 2 > τ > τ 1 at η < 0 for F ↑ (τ). After modelling, the required fluxes are finally determined from the relationships identical to (8.3.16). It should be mentioned that in calculating the fluxes above the orthotropic surface and with the phase function independent of the azimuth, the azmith co-ordinate may be ingored. Physically, this follows from the fact that the model of transfer and calculation of the fluxes (integrals in respect of the azimuth) does not contain processes with azimuthal anisotropy. Mathematically, we have clearly proved this statement, showing that the zenith angles of scattering in the atmosphere and reflection on the surface do not depend on the azimuth ϕ1 of the photon prior to interaction, and only the zenith angle takes part in the modelling of the path of the photon and influences the counting of photons.

8.4. Algorithms and programmes for calculating radiation characteristics of the atmosphere (radiation codes) Here we examine various methods of calculating the transmittance function, intensities and radiation fluxes (thermal and solar) in the 370

Main Concepts of the Theory of Solar Radiation Transfer

atmosphere. These methods were used as a basis for the developing in recent years a large number of algorithms and calculation programmes (radiation codes). In many cases these codes are available for mass application and are used widely for solving different applied and scientific problems – calculating the transmittance functions for different paths of propagation of radiation, calculating the intensity and fluxes for different measurement geometry, etc. We describe the most widely used codes briefly. It should be stressed that we do not aim to provide a complete review of the radiation codes used at present because the number of these codes is very large, and we shall only illustrate them on typical examples [93, 97]. The direct method of calculation was used as a basis for developing codes in many scientific institutions (for example, St. Petersburg State University, A.I. Vavilov State Optical Institute, Institute of Atmospheric Optics of the Russian Academy of Sciences, Institute of Physics of the Atmosphere of the Russian Academy of Sciences, Kurchatov Institute of Atomic Energy, The Geophysical Laboratory of US Air Force, etc.). We describe typical examples of the currently available radiation codes. The code FASCOD-3 (The Fast Atmospheric Signature Code – version 3 ) uses an effective algorithm for direct calculating transmittance functions (for homogeneous and inhomogeneous conditions) and calculating the intensity of radiation in the spectral range from ultraviolet to microwave range (0–50 000 cm –1 ) in the altitude range from 0 to 120 km. In calculating radiation we can take into account the processes of multiple scattering of radiation, the effects of line mixing, deviation from LTE, absorption of oxygen and ozone in ultraviolet and visible ranges of the spectrum. In addition to this, calculations can be carried out for the so called weight functions used when solving inverse problems of atmospheric optics (Chapter 10). Calculations are carried out assuming a Voigt contour of spectral lines. Deviations from Lorentz lines are taken into account using continual adsorption of H 2 O and a correction function for CO 2. The parameters of the spectral lines are selected on the basis of the HITRAN database. The radiation code includes different models of aerosol and clouds. The models of the atmosphere includes six climatological regions of the Earth (including changing profiles of the content of H 2 O, O 3 , CH 4 , CO and NO 2 ) and mean global profiles of 20 other gases of the atmosphere. Calculations may take into account the spectral characteristics of reflection from the underlying surface. The quality 371

Theoretical Fundamentals of Atmospheric Optics

of calculations using the FASCOD-3 radiation code was verified by comparison with calculations using other codes and experimental data. RADTRAN code is designed for calculating atmospheric attenuation and the brightness temperature of thermal radiation in the spectral range 1–300 GHz. Calculations are carried out taking into account the molecular adsorption of oxygen, water vapour (lines and continuum), and extinction in clouds and precipitation. Atmospheric models were taken from LOWTRAN (see later) or are introduced into the code by the user. The geometry of measurements and the emitting properties of the surface are also specified by the user. In addition to this, the code contains the emissivities of nine types of surface (taking into account their polarisation characteristics). Multiple scattering is taken into account when calculating the radiation characteristics in precipitation. GENLN-2 code is used to calculate the transmittance function and the intensity of thermal radiation for different measurement geometries. The refraction in the atmosphere and the specific nature of the atmosphere itself are taken into account. The shape of the spectral lines is of the Voigt type. The deviation of the shape from the Voigt shape for the wings of the lines is taken into account. The parameters of the lines are taken from the HITRAN database. The phenomena of line mixing in the adsorption bands of CO 2 are taken into account. The latest versions of GENLN-2 code can also be used to carry out calculations for a non-equilibrium atmosphere. SHARC code was specially developed for calculating nonequilibrium radiation in the infra-red range of the spectrum (2–40 µm) for the atmospheric altitudes from 60–300 km. The code takes into account the radiation of the five most importance atmospheric gases for the examined altitudes – CO 2 , NO, O 2, H 2O and CO. The code is used for calculating the population of the excited states of these molecules. In addition to this, the code contains a module for calculating the characteristics of the gas composition of the atmosphere. The selectivity of adsorption is taken into account using a model of the isolated line which restricts the limiting spectral resolution of the calculations to the value of 0.1 cm –1 . The shape of the lines of absorption is of the Voigt type. The initial parameters of the lines are taken from the HITRAN database. SPbGU code is designed for calculating the transmittance functions and intensities of radiation in the infrared range of the 372

Main Concepts of the Theory of Solar Radiation Transfer

spectrum taking into account deviations from the LTE conditions. The measurement geometry is on the horizon of the planet. The parameters of the spectral lines are taken from the HITRAN base, the line shape is of the Voigt type. CO 2 line mixing is taken into account. The code contains a block for calculating partial derivatives of the intensity of radiation using different parameters of the physical state of the atmosphere (kinetic temperature, vibrational temperatures, the content of adsorbing gases). Calculations for non-equilibrium conditions are performed using the profiles of vibrational transitions of the appropriate levels of the molecules. The SPbGU code is used mainly for solving inverse problems in the conditions of LTE breakdown. The GOMETRAN code is designed for calculating the intensity of outgoing reflective and scattered solar radiation in the spectrum range 240–270mm. The last versions of the code, for example SCIATRAN, can be used to calculate the outgoing radiation in a longer wavelength range. In particular, GOMETRAN is designed for interpreting the measurements of GOME satellite multi-channel spectrometer (European satellite ERS-2) and, therefore, it can be used to calculate the partial derivatives of outgoing radiation in respect of different parameters of the atmosphere – the content of absorbing gases and the optical characteristics of the aerosol. To solve the equation of radiation transfer in the plane-parallel model of the atmosphere taking multiple scattering into account it is necessary to use the method of discrete ordinates. The approximate calculations of the sphericity of the atmosphere is carried out only taking into account the sphericity when calculating direct solar radiation. The initial version of the code is designed for use in a cloudless atmosphere. This was followed by the development of versions for a cloudy atmosphere. The k method is used to take into account selective molecular adsorption. Additionally, the authors of code GOMETRAN (Bremen University, Germany) developed codes for calculating the outgoing solar radiation for a spherical model (examination of the horizon of a planet). The two following calculation codes do not use the direct method of calculating the radiation characteristics of the atmosphere. LOWTRAN code. There are various versions of this code which have been gradually improved. We shall describe version LOWTRAN-7. The code is used to calculate the transmittance functions and the intensity of radiation for a relatively low spectral resolution (starting at 20 cm –1 ) in a wide spectral range from 0 to 50 000 cm –1 . The intensities of radiation (thermal and solar) can be 373

Theoretical Fundamentals of Atmospheric Optics

calculated taking into account multiple scattering. The calculations are carried out using the transmittance functions for finite spectral intervals in the form of exponential functions. Selective and continual molecular absorption, molecular scattering, aerosol scattering and adsorption, absorption and scattering on clouds and precipitation are taken into account. Calculation can also be carried out for different measurement geometries in a spherical atmosphere taking refraction effects into account. Calculations can be carried out using six mean climatic models of the atmosphere or arbitrary models introduced by the user. Molecular adsorption is taken into account using the transmittance function in the form m   p  T   P∆ν (ν) = exp  −Cνu  0  0   ,  p  T    a

(8.4.1)

where the parameters C, n, m and a are determined for every spectral range 20 cm –1 wide on the basis of approximation of the calculation of the transmittance function for different pressures, temperatures and the content of absorbing gases by the direct method (code FASCOD) for the same spectral ranges. Aerosol models represent a four-layer atmosphere – boundary layer (0–2 km), troposphere (2–10 km), stratosphere (10–30 km) and mesosphere (30–120 km). For the boundary layer there are various types of aerosol models – village, city, sea, tropospheric, desert and ‘naval military’. The models of the clouds includes five types of water clouds and also a number of crystalline clouds. Because of the relatively simple approximation of molecular absorption and approximate consideration of the effect of multiple scattering the code LOWTRAN-7 is characterised by medium accuracy and is usually used for applied calculations. The MODTRAN code was developed as an improved version of the LOWTRAN code and it differs from the latter mainly by the possibility of carrying out calculations with a higher spectral resolution (starting at 2 cm –1 ). In addition to this, the code uses a more advanced approach when taking into account molecular absorption (use of the Voigt line shape, the temperature dependence of the transmission functions, etc.). As regards its quality and possibilities, this code occupies an intermediate position between LOWTRAN and FASCOD codes.

374

CHAPTER 9

RADIATION ENERGETICS OF THE ATMOSPHERE–UNDERLYING SURFACE SYSTEM The influx of heat in the form of radiant energy (energy of electromagnetic radiation) is the most important compound part of the total influx of heat whose effect changes the thermal regime of the atmosphere and of the underlying surface [20, 32, 33, 37, 43, 51, 92, 102, 103]. The radiation balance or the balance of radiant energy of the system is the difference between the radiation absorbed by the system and its natural radiation. In meteorology and physics the atmosphere we examine the radiation balance of difference systems: the surface, atmosphere, the atmosphereunderlying surface system. We start with the analysis of an important component of radiation balance (RB) – the radiation from the Sun on the upper boundary of the atmosphere.

9.1. Solar insolation at the upper boundary of the atmosphere Insolation Q is the flux of solar radiation incident on the unit horizontal area during the given period of time (t 2 –t 1) [32, 37, 79]:

∫

Q = F ↓ (t )dt. t2

(9.1.1)

t1

These fluxes may be examined in the vicinity of the underlying surface, on different levels in the atmosphere, and so on. It should be stressed that these fluxes relate to the entire spectrum of solar radiation. It is natural to start the study with insolation on the upper boundary of the atmosphere because this value determines the energy received from the Sun at different latitudes and different periods of the year. The solar radiation flux at the upper boundary of the atmosphere is determined by the equation

F ↓ (t ) = F0↓ cos θ(t ), 375

(9.1.2)

Theoretical Fundamentals of Atmospheric Optics

where F 0↓ is the flux on the unit area, normal to the propagation of solar radiation, at the upper boundary of the atmosphere; θ is the zenith angle of the Sun at the given point and time. If it is taken into account that the distance between the Earth and the Sun changes during movement of the Earth around the orbit, we may write that F0↓ =

r02 S0 , r2

(9.1.3)

where r 0 and r are the mean and instantaneous distances of the Earth from the Sun; S 0 is the flux of solar radiation, corresponding to the mean distance (the solar constant for the Earth). The relative changes of the solar flux at the upper boundary of the atmosphere d=

F0↓ − S0 S0

are presented in Table 9.1 for different months of the

year. It should be noted that in winter the amount of solar energy received by the Earth in the Northern hemisphere is almost 7% larger than in summer. The total solar energy incoming every day on the unit area may be determined by integration in equation (9.1.3) in respect of ‘light’ time of the day, i.e. from sunrise to sunset:

r2 Q = S0 02 r

∫

sunset

cos θ(t )dt.

(9.1.4)

sunrise

In equation (9.1.4) we ignore the variation of the ratio d during the day. The zenith angle of the Sun may be expressed through other angles – the inclination of the Sun, the time angle h and latitude ϕ: cosθ = sinϕ·sinδ + cosϕ·cosδ·cosh.

(9.1.5)

Substituting equation (9.1.5) into equation (9.1.4) and denoting the angular velocity of rotation of the Earth by ω = 2π/24 = π/12 rad/hour), we obtain Table 9.1. Relative changes (d, %) of the flux F0↓ at the upper boundary of the atmosphere in relation to the month N N

1

2

3

4

d

3.4

2.8

1.8

0.2

5

6

–1.5 –2.8

376

7 –3.5

8

9

10

–3.1 –1.7 –0.3

11

12

1.6

1.8

Radiation Energetics of the Atmosphere–Underlying Surface System

r  Q = S0  0  r

∫ (sin ϕ ⋅ sin δ + cos ϕ ⋅ cos δ ·cos(tω))·dt ,

2 H

−H

(9.1.6)

where H is the half of the light period of the day, i.e. the time from sunrise or sunset to the solar midday. Integrating equation (9.1.6), we obtain Q=

S  r0   1  sin ϕ ⋅ sin δ ⋅ H + cos ϕ ⋅ cos δ ⋅ sin( H ω)  . (9.1.7)    πω  r   ω  2

In equation (9.1.7) the value of H in the second term on the right is expressed in radians (180 = π rad). The results of calculations by equation (9.1.7) of the daily sums of the solar energy, incoming on the unit area at the upper boundary of the atmosphere, in relation to latitude and the day of the year are presented in Fig.9.1. Since the Sun is closest to the Earth in January (the winter in the northern hemisphere), the distribution of daily sums of the solar energy is far from uniform. The southern hemisphere receives more radiation than the northern one. The maximum insolation takes place in the summer at poles because of the duration of the light time of the day (24 h). The minimum amount, equal to zero during polar nights, is at both poles. After integrating equation (9.1.7) in respect of the yearly time period it can be shown that the total annual insolation is the same for the appropriate latitudes of the northern and southern hemispheres.

9.2. Radiation balance of the surface According to definition, the radiation balance of the surface R s is the difference between the radiation absorbed by the surface F a and its natural radiation F e : R s = F a – F e.

(9.2.1)

Absorbed radiation consists of two components – absorbed solar radiation Q(1–A) and absorbed outgoing long-wave radiation of the atmosphere F a↓ (Fig.9.2): F a = Q(1 – A) + F a ↓ ,

(9.2.2)

where Q is the total solar radiation consisting of direct Q d and scattered Q s solar radiation (Q=Q d +Q s ); A is the surface albedo. If the surface is ‘absolutely black’ it absorbs all descending thermal 377

Winter equinox

Sun

Autumn equinox

Summer equinox

Spring equinox

Theoretical Fundamentals of Atmospheric Optics

Fig.9.1. Daily sums of the solar energy incident on the unit area on the upper boundary of the atmosphere in relation to latitude and the day of the year [37].

radiation of the atmosphere F a ↓ (it is often referred to as the downward radiation of the atmosphere) i.e. F a ↓ = F ↓ . If the emissivity of the surface is ε, the surface reflects part of the downward radiation of the atmosphere (1–ε)F ↓ and consequently F a ↓ =εF ↓ . Thus, for the ‘non-black’ surface F = Q(1–A) + εF a ↓ .

(9.2.3)

Since in equations (9.2.2) and (9.2.3) we consider the quantities that are integral in respect of the spectrum, the surface albedo and emissivity also relate to the entire spectrum of solar and thermal radiation, respectively. The flux of natural radiation of the surface F s ↓ is determined by its temperature and emissivity and can be represented as εσ B T s 4 , 378

Radiation Energetics of the Atmosphere–Underlying Surface System

Fig.9.2. Radiation balance of the underlying surface.

where T s is the temperature of the underlying surface. The descending thermal radiation of the atmosphere may be written in the following approximate form

F ↓ = σ BTa4 (1 − Pa ),

(9.2.4)

where T¯ a is the mean temperature of the atmosphere; P a is the integral transmittance function for the entire thickness of the atmosphere (for the radiation flux). In some cases, the radiation balance is subdivided into its shortwave R s–w and long-wave R l–w parts. If we introduce the concept of the effective thermal radiation on the surface Feff = Fs↑ − F ↓ ,

(9.2.5)

Then the radiation balance for the non-black surface may be represented as follows Rs = Rs − w + R1− w = Q (1 − A) − εFeff = = Q (1 −

A) − εσ BTs4

+ εσ BTa4 (1 −

(9.2.6) Pa ).

Calculations and measurements of surface radiation balance The radiation balance of the underlying surface strongly affects its temperature, the distribution of temperature in soil, the ground temperature of the atmosphere, the processes of evaporation and snow melting, formation of mist and light frost, and the processes of transformation of the properties of air masses. The radiation balance of the underlying surface greatly varies and depends on the latitude, the time of year and day, climate conditions, and the properties of the underlying surface. The radiation balance is 379

Theoretical Fundamentals of Atmospheric Optics

RB and its components, kW/m 2

calculated for different time periods – hours, days, months, seasons and years. The methods of calculating the quantities included in the determination of the radiation balance of the underlying surface were described in chapters 7 and 8. As an example, the diurnal variation of the radiation balance, and its short-wave and long-wave components according to the results of observations in a steppe, are presented in Fig.9.3. In daytime, the radiation balance is positive, at night it is negative, i.e. during the day the surface is heated by solar radiation, and during the night it cools down because of thermal long-wave radiation – radiation cooling of the surface. According to the results of observations, the radiation balance passes through zero at a height of the Sun of 10– 15º. The long-wave component of the radiation balance is always negative, i.e. the radiation of the underlying surface is always greater than the radiation of the atmosphere absorbed by the surface (downward radiation of the atmosphere). This is caused by, in particular, by the presence of the ‘transparency window’ in the central infrared part of the spectrum in the atmosphere at a height of 8–12 µm with low values of the absorption coefficient and, consequently, the low downward radiation of the atmosphere. The downward radiation of the atmosphere depends on the content of the absorbing components of the atmosphere (mainly on the content of water vapour), the profile of temperature and most markedly on the cloud conditions of the atmosphere. The maximum downward radiation of the atmosphere (and consequently, the minimum value of the longwave component of the radiation balance) is found in the

h Fig.9.3. Daily course of radiation balance (RB), its short- and longwave components, according to the results of observations in a steppe [43].

380

Radiation Energetics of the Atmosphere–Underlying Surface System

presence of lower stratum clouds. The measurements and calculations of the radiation balance of the underlying atmosphere have been continuing for a long time. As an example, Table 9.2. shows the calculations of the mean latitude distribution of the annual sums of the radiation balance for various surfaces (dry land, ocean) and for the entire surface of the globes [43]. Table 9.2 shows that the annual average values of the radiation balance of the underlying surface are positive, i.e. radiation heating of the surface takes place. The maximum annual values of the radiation balance of the underlying surface are found in the tropical regions of the surface of the ocean. The results of a large number of measurements and calculated data were used to construct maps of the geographic distribution of the radiation balance of the surface. An example of these calculations is shown in Fig. 9.4. Figure 9.4. shows that the annual radiation balance is positive on the entire territory of the globe and changes from values close to zero in the central Arctic (10 kcal/ cm 2 ·year) in the vicinity of the boundary of permanent ice to 80– 120 kcal/cm 2 ·year in tropical latitudes. Nevertheless, the annual sums of the radiation balance may also be negative in regions with constant or very long-term icy or snow covers, i.e. in Arctic and Table 9.2. Mean latitude distribution of the radiation balance of the underlying surfaces (kcal/cm2·year) Latitude, deg

Ocean

Dry land

Mean

70 – 60 N

23

20

21

60 – 50

29

30

30

50 – 40

51

45

48

40 – 30

83

60

73

30 – 20

113

69

96

20 – 10

119

71

106

10 – 0

115

72

105

0 – 10 S

115

72

105

10 – 20

113

73

104

20 – 30

101

70

94

30 – 40

82

62

80

40 – 50

57

41

56

50 – 60

28

31

28

Entire globe

82

49

72

381

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Antarctic regions. Analysis of the map shown in Fig. 9.4 shows that the variation of the radiation balance in going from dry land to ocean takes place abruptly and this is expressed in a break in the isolines in the vicinity of the shorelines. This is caused mainly by the rapid change of the albedo of the underlying surfaces. The values of the albedo of the oceans are considerably lower than the albedo of the dry land. This increases the value of the absorbed total solar radiation and the radiation balance of the oceans. The second reason for the large changes in the radiation balance in going from ocean to dry land is differences in the temperature of these surfaces leading to differences in the long-wave radiation of the surfaces.

9.3. Radiation balance of the atmosphere The incoming part of the radiation balance of the atmosphere R a forms as a result of the absorption by the atmosphere of the longwave radiation of the underlying surface F ↑ s,a and the absorbed direct and scattered solar radiation Q a (Fig. 9.5). Strictly speaking, the incoming part of the radiation balance should include the solar radiation reflected from the surface and absorbed by the atmosphere. Therefore, Q a denotes the entire solar radiation absorbed by the atmosphere. The outgoing part of the radiation balance is determined by the long-wave atmospheric radiation the direction of the surface (downward radiation of the atmosphere) F a ↑ and into cosmic space F a↑ ,∞ . Thus, the equation for the radiation balance of the atmosphere may be written in the form

Ru = Qa + Fs↑,a − Fa↓ − Fa↑,∞ .

(9.3.1)

If the integral transmittance function of the entire thickness of the atmosphere is denoted by P a, then

Fs↑,∞ = (1 − Pa )Fs↑ = (1 − Pa )εσ BTs4 .

(9.3.2)

The sum F ↑∞ =F s↑ P a +F a ↑ ,∞ is the outgoing radiation of the atmosphere–surface system. Taking this into account, we can write the following equation for the radiation balance of the atmosphere: Ra = Qa + Fs↑ − F ↓ − Fs↑ Pa − Fa↑,∞ = Qa + Feff − F∞↑ , Feff

F∞↑

382

(9.3.3)

Fig. 9.4. Example of calculations of the map of the geographic distribution of the radiation balance of the surface [103].

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Upper boundary of the atmosphere

Fig. 9.5. Radiation balance of the atmosphere.

Calculations show that the mean yearly radiation balance of the atmosphere in all latitudes on average per year is negative, i.e. the atmosphere on average ‘cools down’ as a result of radiation transfer of radiation. This is associated with the fact that the absorption of solar radiation by the atmosphere is relatively small and the absorption of the thermal radiation of the surface by the atmosphere does not compensate the atmospheric downward radiation and the radiation outgoing into space. The variation of the radiation balance of the atmosphere with the latitude for the northern hemisphere is shown by the data in Table 9.3. As indicated by Table 9.3, the maximum cooling is observed in tropics, the minimum – in the mean latitudes. This radiation cooling of the atmosphere is compensated by the turbulent influx of heat from the Earth’s surface, and the main, – by the influx of heat as a result of condensation of the water vapour.

Radiation influx In examining the energetics of the atmosphere it is important to study the energy absorbed and radiated not only by the atmosphere as a whole but also by its individual layers. The radiation balance of the individual layers (or levels) of the atmosphere is expressed

Table 9.3. The radiation balance of the atmosphere (annual average) for different latitudes zones (W/m 2) Latitude Ra

0–10

10–20

20–30

30–40

40–50

50–60

60–70

–101

–110

–109

–92

–80

–80

–93

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in the rate of radiation heating because of the absorption of solar radiation or in the rate of radiation cooling as a result of atmospheric radiation. Figure 9.6 shows the results of calculating the profiles of the rates of radiation heating in the cloudless atmosphere as a result of the absorption of solar radiation. Calculations were carried out taking into account the absorption of O 3 , H 2 O, O 2 and CO 2 , multiple molecular scattering and the reflection from the underlying surface (albedo A = 0.15). It may be seen that in the tropical model of the atmosphere the rate of heating of the troposphere is higher than in the model of the mean latitudes. This is explained by the higher content of water vapour in the tropics and by the correspondingly higher absorption of solar radiation. The maximum rate of radiation heating is found at an altitude of approximately 3 km and equals 4 deg/day. For the model of the mean latitude it exceeds 3 deg/day at this altitude. Above this altitude, the rate of radiation heating rapidly decreases as a result of a rapid decrease of the concentration of water vapour with the increase of altitude, and reaches the minimum value at an altitude of 10–20 km. In higher layers of the atmosphere the rates of radiation heating increase as a result of the absorption of solar radiation in the ozone absorption bands. The ozone concentration reaches the maximum

deg/day Fig.9.6. The results of calculations of the profiles of the rate of radiation heating in a cloudless atmosphere as a result of absorption of solar radiation [37]. 1) tropics; 2) mean latitudes, winter.

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value approximately at an altitude of 25–30 km. The rate of radiation heating as a result of the absorption of solar radiation varies greatly and depends on many parameters, mainly on the condition of the atmosphere (the content of absorbing gases, the presence of clouds of different types, the content and nature of aerosol particles), the zenith angle of the Sun (latitude, season, daytime), and surface albedo. To characterise the spatial variability of the rates of radiation heating as a result of absorption of solar radiation by ozone and molecular oxygen, we give Fig. 9.7. The graph shows that meridional distribution of the rates of heating of the upper atmosphere from 20 to 100 km. The graph shows, for example, that these rates in the summer months have two regions of maximum values – close to the altitude of 50 km, where they reach 18 deg/day, and close to 100 km where they reach 40 deg/day. In the former case, heating is caused by the absorption of ozone, in the second case – by the absorption of oxygen. In winter, the rate of radiation heating is considerably lower than in summer. If the solar radiation heats the entire Earth atmosphere, the role of infrared radiation is more complicated. Thermal radiation mainly cools down the atmosphere because it carries the energy to the

Summer

Winter

Fig.9.7. Meridional distribution of the rate of heating of the upper atmosphere from 20 to 100 km [8].

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cosmic space by outgoing radiation. However, under certain conditions, and at certain levels in the atmosphere and in certain regions infrared radiation may heat the atmosphere. Radiation cooling of the atmosphere is caused mainly by the radiation of water vapour, carbon dioxide and ozone. Figure 7.5 showed the examples of calculations of the rates of radiation changes of the temperature of the atmosphere for the absorption bands of ozone and carbon dioxide. The zone-averaged meridional distributions of the rates of radiation changes of temperature for January and July are shown in Fig. 9.8. Maximum cooling is found in the summer stratosphere and is determined exclusively by the transfer of radiation in the bands of ozone and carbon dioxide. This is caused by the fact that the contribution of water vapour at these altitudes is very small because of the low concentration of water vapour at these altitudes. In addition to this, the ozone is responsible for slight heating of the stratosphere by infrared radiation. This effect is clearly visible in tropical and sub-tropical latitudes above the tropopause in both summer and winter. This heating is caused by the increase of the ozone concentration with the altitude in the stratosphere and by low temperatures in the region of the tropopause. The rates of radiation changes of temperature as a result of the transfer of infrared radiation depend mainly on the thermal structure of the atmosphere, the amount of absorbing gases, and the presence of clouds of different type. Figure 9.9 shows the meridional distribution of the zone-averaged total rates of radiation changes of temperature (as a result of solar and infrared radiation) for January and July. Calculations were carried out taking into account climate models of the Earth atmosphere with clouds also taken into account. Infrared cooling exceeds solar heating in almost the entire atmosphere. In the upper stratosphere, above approximately 25 km, there is strong cooling (4–5 deg/day) as a result of radiation of CO 2 and O 3 whose magnitude is higher than heating as a result of the absorption of solar radiation by ozone. Cooling in the troposphere is associated mainly with the radiation of water vapour and is maximum in the tropics. The presence of clouds (see later) results in slow cooling at the lower levels of the atmosphere. The effect of the clouds changes with the latitude and season because of their variability. The region of slow heating extends from the summer polar regions to tropical latitudes in the winter hemisphere on the levels of approximately 5 km and is associated with the absorption of solar radiation by water vapour. 387

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Altitude, km

a

80 o N

80 o S

80 o N

80 o S

Altitude, km

b

Latitude Fig.9.8 Zone-averaged meridional distribution of rates of radiation variations of temperature in the absence (1) and presence (2) of aerosols [37] for January (a) and July (b).

This special feature is also caused by heating as a result of the presence of clouds. The maximum values of heating are found in the effect caused by the continuous solar illuminated of the atmosphere is stronger than the effect of large solar zenith angles. Infrared cooling in the same regions is relatively small because of the low temperature of the atmosphere and the effect of clouds. In both hemispheres, the maximum cooling is found in the boundary layer in the region of the tropics in the winter. This is associated with a high content of water vapour in the vicinity of the surface and partially with the relatively small amount of clouds in the tropics in winter in comparison with the amount of clouds in summer.

Effect of cloudiness on radiation balance The clouds in the Earth’s atmosphere cover on average on 50% of 388

Radiation Energetics of the Atmosphere–Underlying Surface System

Altitude, km

a

Altitude, km

b

Southern latitude

Northern latitude

Northern latitude

Latitude

Southern latitude

Fig.9.9. Meridional distribution of the zone-averaged total rates of radiation changes of temperature (as a result of solar and infrared radiation) for January (a) and July (b) [37].

the surface of the globe. They are a powerful ‘regulator ’ of the radiation balance of the surface and of the atmosphere itself. The presence of clouds greatly increases the reflection of solar radiation into space. Usually, the albedo of the clouds is greater than the albedo of the oceans and land. An exception is surfaces covered with snow and ice. This effect, referred to in the English literature as the effect of solar albedo, decreases the amount of solar energy available for transformations in the atmosphere–surface system and results in cooling of this system. On the other hand, the clouds reduce the amount of thermal radiation outgoing into space as a result of the absorption of thermal radiation by the Earth’s surface and the layers of the atmosphere below the clouds. This decrease is associated with the fact that in this case the outgoing radiation forms at lower 389

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temperatures detected at the upper boundaries of the clouds. This effect is referred to as the greenhouse effect and increases the radiation balance of the surface and the atmosphere and causes heating of the atmosphere–surface system. The resultant effect of clouds on radiation balance strongly depends on the horizontal and vertical characteristics of the clouds, their phase state, the content of liquid or solid phase of water, and the size distribution function of cloud particles and temperature of the clouds. For the quantitative characterization of the effect of clouds on the rate of radiation change of the temperature of the atmosphere we present the results of calculations (Figs. 9.10 and 9.11) Figure 9.10 shows the vertical profiles of the rates of radiation cooling as a result of transfer of infrared radiation for a cloudless atmosphere and atmosphere with the clouds of the upper, middle and lower strata for the standard model of the atmosphere. The clouds are distributed between the levels: for the upper stratum – 200– 450 mbar for the middle one – 450–735 mbar, and for the lower one – 745–950 mbar. The position of the clouds are shown in the figure. The absorption of H 2 O, CO 2 and O 3 was taken into account. The rates of radiation cooling for the cloud conditions are given for the cloud amounts of up to 100%. In the cloudless atmosphere, the rate of infrared cooling is maximum (~2 deg/day) near the Earth surface. The rate decreases with increase of altitude to ~12 km and then increases because of absorption and emission in the bands of carbon dioxide and ozone. In the presence of the clouds of a higher stratum the maximum rate of cooling (~4 deg/day) is found at the upper boundary of the clouds. There is also small heating in the vicinity of the base of the clouds. In the case of the clouds of the middle belt there is strong cooling (~11 deg/day) at the upper boundary of the clouds and intensive heating (~4 deg/day) at their base. Slow heating also takes place in the adjacent layer of the atmosphere in the vicinity of the lower boundary of the clouds of the middle stratum. A similar pattern is also detected for the clouds of the lower stratum. The graphs show clearly that the presence of the clouds greatly changes the vertical profiles of infrared radiation changes of temperature. In this case, the main special features are the strong cooling of the upper part of the clouds and intensive heating of the base of the clouds. The rates of heating as a result of the absorption of solar radiation is shown in Figure 9.11. Calculations were carried out for 390

Radiation Energetics of the Atmosphere–Underlying Surface System a

b

c

d

Rate of variation of temperature, deg/day

Rate of variation of temperature, deg/day

Fig.9.10. Vertical profiles of the rates of radiation cooling as a result of transfer of infrared radiation for a cloudless atmosphere (a) and an atmosphere with clouds of the upper (b), middle (c) and lower (d) stratum for the standard model of the atmosphere [103].

a solar constant of 1365 W/m 2 , the surface albedo of 0.2 and the cosine of the solar zenith angle of 0.8. The heating rates in the cloudless atmosphere are on average smaller than ~1.5 deg/day. In the presence of the clouds of the upper belt the rate of heating is ~2 deg/day at the upper boundary of the clouds. For the clouds of the middle and lower stratums, these rates are ~8 deg/day at the upper boundary of the clouds.

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Theoretical Fundamentals of Atmospheric Optics

a

b

c

d

Heating rate, deg/day

Heating rate, deg/day

Fig.9.11 Vertical profiles of the rates of radiation heating as a result of the absorption of solar radiation [103]. (a) Cloudless atmosphere; (b) clouds of the upper stratum; (c) clouds of the middle stratum; (d) clouds of the lower stratum.

9.4. Radiation balance of the planet Study of the radiation balance of the entire atmosphere–underlying surface system is of considerable interest. This balance characterises the balance of the radiation energy in the vertical column, including the active layer of the surface and the entire atmosphere. In other words, the radiation balance of the Earth as the planet is discussed here. ‘The input part’ of this balance consists of the direct and scattered solar radiation absorbed by the Earth’s surface and the atmosphere, and ‘output’ part is the outgoing longwave radiation: 392

Radiation Energetics of the Atmosphere–Underlying Surface System

R = Q(1 – A) + Q a – F ∞ ↑ .

(9.4.1)

Equation (9.4.1) is obtained as the sum of radiation balances of the surface (9.2.6) and the atmosphere (9.3.3). The equation for R can also be written in the form R = Q ∞ (1 – A n ) – F ∞ ↑ ,

(9.4.2)

where Q ∞ is the mean (per annum) flux of the direct solar radiation at the upper boundary of the atmosphere; A n is the albedo of the Earth as the planet. The value of the mean flux of the solar radiation Q ∞ is easy to calculate. Infact, the amount of solar energy coming to the Earth in unit time is equal to the product of the solar constant S 0 by the area of the cross section of the Earth πR 2 (R is the mean radius of the Earth). This value is distributed over the entire surface of the globe and equal to 4πR 2 . Thus, the mean value of the flux of solar radiation per unit of the horizontal surface of the Earth at the upper boundary of the atmosphere is 1/4 S 0 . The first term in the right-hand part of equation (9.4.2) is the solar radiation absorbed by the entire atmosphere–underlying surface system. It should be stressed that here A is not the albedo of the surface and it is the albedo of the entire atmosphere–underlying surface system, i.e. the albedo of the planet. Investigations of the radiation balance (RB) of the planet and of its components have been carried out using calculation method and special apparatus on board of satellites. These satellite devices can be used to measure the solar constant S 0 , the component of solar radiation reflected and scattered by the atmosphere–underlying surface system, characterised by the albedo of the planet A, and outgoing long-wave radiation. Investigations of the components of the radiation balance of the Earth using artificial satellites have been conducted for a long period of time starting in 1959 (TIROS satellite). The polar satellites make it possible to study, during a relatively short period of time, the entire globe and the geostationary satellites can be used for the almost continuous studying of the radiation balance in specific geographical regions (tropical and middle latitudes). A relatively high stability of the climate of the Earth indicates that the heat losses averaged over the globe as a result of the outgoing long-wave radiation balance approximately the absorbed solar radiation. The mean global albedo, according to current estimates, is approximately 30%, the mean flux of the outgoing longwave radiation is 229 W/m 2 . 393

Theoretical Fundamentals of Atmospheric Optics

RB, W/m 2

F ∞↑, W

Albedo, %

Figure 9.12 shows seasonal variations of the global albedo of the planet, the outgoing thermal radiation and radiation balance of the Earth. These data were obtained on the basis of 48 average monthly maps constructed using satellite measurements. The mean monthly albedo of the planet has a maximum value (approximately 32%) during the December solstice. The seasonal variations are partially connected with differences in the surfaces of land and the ocean, in the cloud cover and in distribution of the snow and icy covers for northern and southern hemispheres. Seasonal variations F ∞ ↑ show a maximum in the close to June–July. This is associated with the large areas of the dry land ion the northern hemisphere and with high temperatures of the dry land in the summer months. Thus, the global mean temperature of the surface of the globe is maximum in July (~16.7 ºC) and minimum in January (~13.1 o C). The seasonal variations of the surface temperature control the seasonal dependence of the outgoing thermal radiation. In regions with great humidity, another factor determining the seasonal dependence of F ∞↑

Months Fig.9.12. Seasonal variations of global albedo of the planet, outgoing thermal radiation and Earth’s radiation balance [103]. Dotted curve – average monthly deviations from average yearly insolation for S 0 = 1.376 W/m 2. 394

Radiation Energetics of the Atmosphere–Underlying Surface System

is the seasonal variation of the cloud amount. The lower part of Figure 9.12 shows the seasonal behaviour (in the form of deviations from the mean annual value) of solar insolation at the upper boundary of the atmosphere caused by changes in the distance between the Earth and the Sun. The seasonal variation of the radiation balance of the Earth repeats the seasonal dependence of solar insolation. It should be mentioned that the maximum difference in the solar insolation (January–July) reaches 22 W/m 2 . The seasonal variations of solar radiation, absorbed by the atmosphere–surface system are approximately half of the variations of solar insolation. This is associated with the increase of the albedo of the planet with increase of solar insolation. The seasonal variations of the outgoing thermal radiation of the same order as the variations of the absorbed solar radiation, but they are shifted by 180º in phase. The latitude variations of the mean annual and seasonal (winter and summer) radiation balances of the atmosphere–underlying surface system are shown in Fig. 9.13. The mean annual radiation balance is approximately symmetric in the northern and southern hemispheres with the maximum value at the equator. This maximum is associated with the minimum values of the outgoing long-wave radiation in the tropics caused by the presence of heavy cumulus clouds, and with the strong absorption of solar radiation in the equatorial regions. This special feature is most clearly visible in Fig. 9.14 which shows the latitude distributions of the zone-averaged values of absorbed solar radiation and outgoing long-wave radiation. The negative radiation balance in the polar regions is associated with the fact that the values of the outgoing thermal radiation are considerably greater than the absorbed solar radiation. The low values of absorption are caused by the high values of the albedo of snow and ice. The latitude variations of the radiation balance are also caused by changes of the mean zenith angles of the Sun with latitude. In the winter period the radiation balance has a maximum in the vicinity of 30ºS, whereas in the summer in the vicinity of 15– 20ºN. The global maps of the mean values of the outgoing long-wave radiation, the albedo of the planet and the radiation balance of the atmosphere–surface system, obtained on the basis of satellite measurements over a period of five years (1979–1983) are shown in Fig.9.15 [103]. The values of the albedo show large variations at the dry land–ocean boundaries in the latitude band 30ºN–30ºS as a result of the presence of high convective clouds with a high 395

RB, W/m 2

Theoretical Fundamentals of Atmospheric Optics

s.l.

n.l. Latitude

Q, F ∞ ↑ , W/m 2

Fig.9.13. Latitudinal variations of the mean annual (1) and seasonal (winter and summer) (2,3) radiation balances of the atmosphere–underlying surface system [103].

s.l.

n.l. Latitude

Fig.9.14. The latitudinal distributions of the zone-averaged values of absorbed solar radiation (1) and outgoing long-wave radiation (2) [37].

albedo and low values of thermal radiation. In the ranges extending from 30ºN and 30ºS to geographic poles, the radiation balance is relatively zone-homogeneous, especially in the southern hemisphere. In low latitudes there are individual regions of the input and output of radiation energy. Large variations in the radiation balance are detected in tropical and subtropical regions where the region of desserts in Africa and Arab lands is characterised by negative or slight positive anomalies. In the regions close to Asia with high intensity convective processes there are high positive values of the radiation balance. On the whole, the correlation coefficient between the albedo of the system and the outgoing long-wave radiation is 396

Radiation Energetics of the Atmosphere–Underlying Surface System

a

b

c Fig.9.15 Global map of the mean values of outgoing longwave radiation (a), the albedo of the planet (b), radiation balance (c) of the atmosphere–surface system determined on the basis of satellite measurements over a period of 5 years (1979–1983). 397

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negative mainly as a result of the effect of clouds on these values. An exception is the regions of the desert where the number of clouds is minimum and the surface is strongly reflective and is warm. For the latitude greater than 40º the radiation balance of the planet is basically negative. This sink of radiation energy increases at approach to the poles. The radiation balance of the planet is determined mainly by the fields of temperature and cloud cover. The components of the radiation balance depend on the time of day. The adsorbed solar radiation and the albedo change during the day as a result of the dependence of the processes of scattering and absorption on the zenith angle of the Sun and also on the atmospheric state – mainly of the changes with time on the type and amount of clouds. The outgoing long-wave radiation changes during the day as a result of daily variations of the amount and types of clouds, humidity and the content of absorbing gases and the aerosol and the temperature stratisification of the atmosphere. The quantitative characterisation of the effect of clouds is carried out using the special parameter of the cloud radiation effect [103]. If the outgoing long-wave radiation of the atmosphere–underlying surface system in the conditions of partial cloud cover is represented as the sum of the radiation for the cloudless F 1 and cloudy F 2 atmosphere: F = (1 – n)F 1 + nF 2 .

(9.4.3)

where n is the amount of the clouds, the parameter of the cloud radiation effect C is determined from the equation C = F – F 1 = n(F 2 – F 1 ). In some cases, this parameter is determined separately for solar C s and thermal C e radiations, respectively. The global mean annual values of C s and C e are equal to 48 and 31 W/m 2 , respectively, which correspond to the parameter of the total radiation effect of the clouds of C = –17 W/m 2 .

Global radiation balance We study the global radiation balance of the atmosphere– underlying surface system determined by calculations. The main input parameters in the calculations of the global radiation balance of the atmosphere–underlying surface system are the vertical profiles of the different characteristics of the atmosphere (temperature, gas 398

Radiation Energetics of the Atmosphere–Underlying Surface System Losses through thermal infrared radiation 70

+23

emitted by cloudless atmosphere 34

34

36

+5

+22

–6

absorbed by Earth 44

–115

+33

+67

latent heat 23

turbulence convection

cloudy atmosphere 67

emitted by Earth 1 1 5

transmitted by clouds 22

absorbed by clouds 4 transmitted directly by atmosphere 5

transmitted by clouds 23

absorbed by cloudless atmosphere 22

refle c clou ted by ds 1 7 reflected by earth 6

by ted rcep inte uds 43 clo

by cted refle less cloud here 7 sp atmo

inte by r c e p t e c a t m loudle d ss osp h 5 2 ere

17 6

emitted by cloudy atmosphere 36

Planetary albedo 30

emitted by cloudless atmosphere 33

Incoming solar radiation 100

–29

Lost by Earth 44

Fig.9.16 Analysis of the global radiation balance of the atmosphere–underlying surface system determined by calculations [37].

and aerosol composition), the geometrical and physical properties of the cloud cover, the global amount of every cloud type (lower, middle and upper strate), the albedo of the Earth’s surface, the duration of solar radiance, and the zenith angle of the Sun. The results of analysis for the global mean state of the atmosphere are presented in Fig.9.16. Figure 9.16 shows firstly the distribution of solar radiation in the atmosphere, secondly the distribution of thermal infrared radiation, thirdly the contribution of non-radiation processes to energy transfer (turbulence, convection and condensation of water vapours). The radiation, averaged per annum, coming at the upper boundary of the atmosphere from the Sun, is regarded as 100 %. This solar radiation is sub-divided into three components: the component propagating in the cloudless atmosphere (52%), the component propagating in the cloudy atmosphere (43%) and the component directly arriving on the underlying surface (5%). From the incoming solar radiation, 26% is absorbed by the atmosphere, and 22% – in the conditions of the cloudless atmosphere (absorption by

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atmospheric gases and aerosol) and 4% – by clouds. The Earth’s surface absorbs 44%. The total absorption of solar radiation by the atmosphere–underlying surface system is 70% and 30% of solar radiation is reflected back into the cosmic space, including 7% reflected by the cloudless atmosphere, 17% reflected by clouds, 6% – by the surface of the Earth. At the same time, the atmosphere and the underlying surface generate their own thermal radiation. The outgoing thermal radiation is 34% for the cloudless atmosphere 36% for the cloudy atmosphere which gives a total of 70%. Thus, the losses of energy by the system as a result of outgoing thermal radiation are equal to the absorbed solar radiation. The upward flux of thermal radiation on the level of the Earth’s surface in the units used here is 115%. The downward radiation of the atmosphere =100% (downward radiation of the cloudless atmosphere 33%, downward radiation of the cloudy atmosphere 67%). Thus, the effective radiation flux at the surface is 15%. Adding up together (taking the sign into account) the thermal radiation fluxes coming in and leaving the atmosphere we obtain that as a result of thermal radiation the atmosphere losses 55% of radiant energy incoming from the Sun. If it is taken into account that the atmosphere absorbs only 26% of the arriving solar radiation, the radiation losses of the atmosphere are 29%. These loses are compensated by the generation of heat as a result of the condensation of water vapour evaporated from the surface (the latent heat flux) and by the heat flux from the underlying surface as a result of turbulence and cellular convection. These upward heat fluxes equal 23 and 6%, respectively.

9.5. Radiation factors of climate changes Climate and its changes The most convincing confirmation of changes in the climate of the Earth is Ice Ages – gradual oncoming and disappearance of ice cover in moderate latitudes observed in the last million years. It should be mentioned that the last Ice Age ended approximately 10 thousands years ago. The past climates of the Earth are studied in a special section of climatology – paleoclimatology.

Climate change factors Changes in the climate detected in the last decades and manifested in different manners – the increase of the mean global temperature of the Earth, the decrease of the polar ‘ice caps’ and the rising of

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the level of the world’s oceans – have a strong effect on different aspects of the life of humans and functioning of various branches of economy. This has been the main reason for special interest paid to the problems of the climate of the Earth in the last couple of decades. The task of forecasting climatic changes is one of the priority tasks in modern science. To solve this problem, new models of the climate have been developed and are being developed on the basis of the theory of the climate which is very complicated because of a large number of factors which may influence the climate of the planet and the complexity of the quantitative description of different physical, chemical, and biological processes, controlling the state of the planet. In the group of the factors of climate changes one can mention astronomical reasons (Chapter 1), the drift of the continents, changes in the position of the geomagnetic pole, and many others. For the relative short-term (several hundreds of years) forecasting of climate changes, the most important factor, according to the current views, is the radiation factors – special features of the distribution of radiation balance over the Earth surface. As shown previously, they are related with special features of the composition of the atmosphere: the content of the gases and aerosols absorbing radiation, characteristics and amount of clouds, the properties of the underlying surface. In particular, it should be stressed that the climate of the Earth is determined to a large degree by the natural greenhouse effect (GE) [95]. This effect is caused by the relatively high transparence of the atmosphere for short-wave solar radiation and considerable absorption of the long-wave radiation by the surface of the Earth by various atmospheric gases such as water vapour, carbon dioxide, ozone and clouds. Because of this absorption, a large part of the radiation of the surface does not escape to the cosmic space and heats the atmosphere. If there was no absorption of long-wave radiation then, according to simple estimates, the mean temperature of the Earth would be approximately 255 K. The mean temperature of the Earth’s surface is 20–30 K higher than this temperature. When examining the Earth’s climate there are two important problems – the time variation of the natural GE and the effect of antopogenic activity on the natural GE. In analysis of these problems it is conventional to sub-divide the processes determining the GE of the atmosphere–surface system, into two classes: – the forcing processes regarded as the external effects on the climatic system; – feedback processes in the climatic system. 401

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The processes of the effect include changes in the content of optically active gases (CO 2 , methane, etc), natural and antopogenic aerosols, the products of volcanic eruptions and the solar constant. Direct consequences of these effects are changes of the temperature in the system and its circulation. The temperature changes may lead to other changes of the radiation properties of the atmosphere and the surface. For example, an increase of temperature can increase the evaporation of the water vapour from the surface of oceans. An increase of the content of water vapour in the atmosphere (the most important greenhouse gas) can result in a further increase of the temperature of the system. This example of the forcing illustrates a positive feedback. Other feedbacks may be associated with changes of the amount, height and type of clouds, the planetary albedo with a result of changes in the snow and ice cover, and with changes of the vegetation cover and the albedo of land. Finally, the relatively slow but very large potential changes in the heat fluxes and the accumulation of energy by the oceans should be considered as a feedback. It should be mentioned that the separation of the processes into the forcing processes and the feedback processes is not always unambiguous. For example, changes in the albedo of land may be interpreted as the feedback processes and the processes of the effect on the climatic system, taking into account, for example, changes in the albedo of land as a result of forest cutting. Suitable examples of different effects and also feedbacks in the climatic system are numerous and this in particular explains the difficulties in the simple evaluation of current changes in the Earth’s climate. Another reason for difficulties and indeterminacy of these estimates is the fact that many processes, including radiation processes, have been studied insufficiently. For example, insufficient attention has been given to examining the processes of absorption of solar radiation by different types of clouds. Estimates of this component of the radiation balance of the atmosphere vary from 3 to 10%.

Effect of greenhouse gases Changes in the composition of the atmosphere were discussed in Chapter 2 where it was noted that they may be caused by both natural and antropogenous reasons. For example, the agricultural activity of mankind increases the content of carbon dioxide in the atmosphere. Thus, the problem of climate forecasting is complicated by the need to separate natural reasons for changes in the climate 402

Radiation Energetics of the Atmosphere–Underlying Surface System

from antropogeneous reasons which may be controlled. In the atmosphere of the Earth there are also changes of other atmospheric optically (greenhouse) gases in the infrared range of the spectrum – methane, N 2 O, different freons. An increase in the content of these gases will lead to the greenhouse effect. Table 9.4 gives the characteristics of contributions of different atmospheric gases to the greenhouse effect. In addition to the greenhouse effect (per one molecule in relation to one molecule of CO 2) , Table 9.4. gives information of the mean mixing ratios of gases (in fractions of 10 –9 ) in 1992, and also the possible effect on the absorption of radiation and the gas composition of the atmosphere. The data show that many trace gases are more effective in the influence on the greenhouse effect in the atmosphere (per molecule!) than the ‘conventional’ greenhouse gas CO 2 . To characterise the effect of changes in the composition of the greenhouse gases in the atmosphere, we present estimates of changes in the surface temperature caused by corresponding changes of the content of these gases (Table 9.5). These estimates were obtained using a relatively simple one-dimensional model of the atmosphere disregarding feedbacks in the climatic system. For comparison, Table 9.5 also gives the changes in the surface temperature caused by a decrease of the ozone content, a 2% increase of the solar constant and the addition of the stratospheric layer of the aerosol with the optical thickness of 0.15. The data in Table 9.5 show that the main greenhouse gas (with the exception of water vapour) is the carbon dioxide and the change of the content of carbon dioxide may lead to considerable climatic changes.

Water vapour, clouds and precipitation As already mentioned, the water vapour is the main greenhouse gases in the Earth’s atmosphere and an increase of the temperature of the atmosphere–surface system can increase the extent of evaporation of water from the surface of the oceans, the humidity of the atmosphere and the greenhouse effect of water vapour. However, in addition to this, an increase of atmospheric humidity may result in an increase of the amount of clouds because of the condensation of water vapour in the atmosphere. The increase of the amount of clouds influences the radiation balance in two ways: on the one hand, the reflection of the solar incoming radiation increases and, on the other hand, the outgoing thermal radiation of 403

Theoretical Fundamentals of Atmospheric Optics Table 9.4. Efficiency of greenhouse effect induced by various gases [99]

Gas

Mixing ratio in 1992, ppb

Efficiency of GE*

O3

10–200

CO2

356000

1

CH4

1714

21

Comments Absorbs UV and IR radiation Absorbs IR radiation, affects stratospheric ozone Absorbs IR radiation, affects tropospheric O 3 and OH, stratospheric O 3 and H 2O

N 2O

311

206

Absorbs IR radiation, affects stratospheric ozone

CFCl 3 (CFC–11)

0.268

12400

As above

CF 2 Cl 2 CFC–12)

0.503

15800

As above

CH 2 HCl (HCFC–22)

0.105

10600

As above

CH 3 CCl 3

0.160

2730

As above

CF 3 Br (H–1301)

0.002

16000

As above

* Greenhouse effect is given in relation to the GE of carbon dioxide Table 9.5. Variations of surface temperature [99] Mechanism of the effect

T, deg

Increase of content of CO 2 (300 – 600 ppm)

1.31

Increase of content of CH 4 (0.25 – 0.56 ppm)

0.16

Increase of content of N 2 O (0.16 – 0.32 ppm)

0.27

Increase of content of CFC-11 (0 – 1 ppb)

0.07

Increase of content of CFC-12 (0 – 1 ppb)

0.08

50% decrease of ozone content at all heights

–0.38

Increase of solar constant by 2%

1.35

Addition of the stratospheric layer of aerosol with the optical thickness of 0.15

–0.99

the atmosphere–surface system decreases. We discussed these effects in paragraph 9.4 and it was shown that the overall effect in the mean may result in cooling of the climatic system. However, in individual situations, for example, in regions with high values of 404

Radiation Energetics of the Atmosphere–Underlying Surface System

the albedo of the underlying surface (snow or ice) the overall effect is equal to zero or may even result in heating the climatic system. The processes of condensation of water vapour and of precipitation on the surface of the Earth are important factors of climate regulation. An increase in the evaporation from the surface of the ocean, and an increase in the intensity of the processes of condensation of water vapour and the formation of clouds change the amount of heat coming into the atmosphere. Precipitation, especially in the form of snow, can greatly change the albedo of the surface and, consequently, its radiation balance.

Atmospheric aerosol Study of the data obtained in the ground-based measurements of direct and scattered solar radiation shows that the amount of shortwave radiation coming on the surface in the conditions of a cloudless atmosphere on the surface, greatly changes from year to year. The main reason for these changes is the large changes in the content of aerosol particles in the atmosphere. Aerosol attenuates the solar radiation incident on the surface, increases the intensity of the processes of scattering in the atmosphere, including backscattering, i.e. can increase the component of the solar radiation reflected into space. If the aerosol is absorbing, this increases the absorption of solar radiation in the atmosphere. To a lesser degree but still noticeably the aerosol can influence the thermal radiation fluxes. For example, the scattering and, especially, the absorption of infrared radiation by the aerosols reduce the intensity of outgoing thermal radiation. The overall effect of the aerosol is greatly changeable and this is associated with large changes in the concentration of aerosol particles and their microphysical and, consequently, optical properties. On the average, according to current views, an increase of the content of aerosol in the atmosphere results in cooling the climatic system. The stratospheric aerosol plays an important role in impacting the climate (Table 9.5). Its concentration in the stratosphere can increase by several orders of magnitude as a result of strong volcanic eruptions. It is therefore assumed that the eras of increased volcanic activity of the Earth correspond to the eras of cooling down.

Changes in the characteristics of the underlying surface Changes in the radiation properties of the underlying surfaces can 405

Theoretical Fundamentals of Atmospheric Optics

also greatly affect the radiation balance of both the surface and the entire planet. Consequently, natural and antropogenous changes of the surfaces can be responsible for changes in the climate of the Earth. These changes can be manifested both as the forcing processes (for example, antropogenous – cutting down forests, agricultural activities, growth of large cities) and feedbacks. In both cases, the albedo of the underlying surfaces changes. The processes of feedback in the climatic system include processes such as the changes of the area covered by snow and ice as a result of the warming of the climate, increasing the size of deserts, rising the level of the world oceans and flooding the areas of land. An increase of the albedo of the underlying surface or of the planetary albedo results in the increase of the reflection of incidence solar radiation into space and, consequently, may result in cooling the climate of the Earth.

406

CHAPTER 10

RADIATION AS A SOURCE OF INFORMATION ON THE OPTICAL AND PHYSICAL PARAMETERS OF ATMOSPHERES OF PLANETS 10.1. Direct and inverse problems of the theory of transfer of radiation and atmospheric optics In the theory of transfer of radiation and atmospheric optics we are concerned with two types of problems which are referred to direct and inverse problems [47, 71]. The direct problems include determination of characteristics of radiation. In inverse problems the available characteristics of radiation are used to determine the optical or physical parameters of the atmosphere or underlying surfaces. It should be mentioned that the concepts of direct and inverse problems are typical of many sections of mathematical physics. The concepts of direct and inverse problems of mathematical physics are based on the directivity of the investigated causal connections. Direct problems are oriented in the direction of the course of the causal connection, i.e. they represent problems of determining the effect of known causes. They include the problems of determination of space–time fields for given sources, calculation of the device response to a known signal at the input, and so on. The inverse problems are associated with the conversion of the causal connection, i.e. finding unknown causes from known effects – determination of the characteristics of the sources of the field in some points or regions of the space on the basis of the results of measurement of the parameters of the fields, retrieval of the input signal on the basis of a response at the output of a device, and so on [75]. The simplest example of an inverse problem will be discussed. The Bouguer law describes the processes of extinction of radiation during its propagation in the atmosphere. The simplest characteristic of such extinction is the optical thickness of the investigated layer. Knowing the incoming I 0 and outgoing I radiation 407

Theoretical Fundamentals of Atmospheric Optics

for this layer, it is easy to determine an important optical characteristic – the optical thickness of the layer: I  τ = 1n  0  .  I

(10.1.1)

Equation (10.1.1) is the simplest example of solving inverse problems of the theory of radiation transfer. If we know the physical reasons for the observed weakening of radiation, for example, it is caused by natural absorption, we can write an explicit expression for the optical thickness of the layer: τ = ku, where k is the absorption coefficient, u is the amount of absorbing substance. If we know the values of the absorption coefficient, for example, if these values were measured previously in laboratory conditions, the next simple step is the determination of the amount of absorbing substance u. This is an example of solving an inverse problem of atmospheric optics. Remote methods have been used successfully for a long time in the study of the atmosphere of the Earth and other planets. In his monograph Twilight [61], G.V. Rosenberg described an interesting example, possibly one of the first remote measurements of the atmospheric characteristics. The Middle Ages philosopher and scientist El Hasan determined the vertical dimensions of the Earth’s atmosphere (52 thousand steps) analysing twilight phenomena. At present, in investigations of the Earth the remote methods are used both in observation from space and from the surface of the Earth or using different flying systems. The advanced remote methods of measurement use the measurement of radiation in a very wide spectral range – from ultraviolet to radiowave range. All remote methods of measurements are inverse problems of atmospheric optics. However, we show that the inverse problems of atmospheric optics is a more general concept [71]. In atmospheric optics in solving direct and inverse problems we are concerned with the following quantities and characteristics (Table 10.1): a ) characteristics of the radiation field (it will be denoted by symbol J); b) parameters of the physical state of the medium (X); c ) parameters of the interaction of radiation with the medium (optical parameters) (A); d) boundary conditions (G); e ) geometry of the examined medium (S). 408

Radiation as a Source of Information on Optical and Physical Parameters Table 10.1. Main characteristics used on atmospheric optics [71] P a ra me te r o r c ha ra c tristic C ha ra c te ristic s o f the fie ld o f e le c tro ma gne tic ra d ia tio n P a ra me te rs o f the p hysic a l sta te o f the me d ium

P a ra me te rs o f the inte ra c tio n o f ra d ia tio n with the me d ium (o p tic a l p a ra me te rs) Bo und a ry c o nd itio ns

Ge o me try o f the me d ium

Exa mp le s S to k e s ve c to r – p a ra me te r, ra d ia tio n inte nsity, e tc Te mp e ra ture o f a tmo sp he re a nd und e rlying surfa c e , c o nc e ntra tio n o f a b so rb ing, sc a tte ring a nd e mitting mo le c ule s a nd a e ro so l p a rtic le s, sp e e d o f wind , mo isture c o nte nt o f so il, e tc Einste in c o e ffic ie nts, c o e ffic ie nts o f a b so rp tio n, sc a tte ring a nd ra d ia tio n, sc a tte ring p ha se func tio n, e tc S o la r ra d ia tio n a t the up p e r b o und a ry o f the a tmo sp he re , ra d ia tio n o f the und e rlying surfa c e , e tc P la in- p a ra lle l a tmo sp he re , sp he ric a l a tmo sp he re , e tc

N o ta tio n J X

A

G

S

When solving the direct problems of atmospheric optics it is assumed that the parameters of the physical state of the medium and the parameters of interaction of radiation with the medium are known together with the boundary conditions and geometry of the system, and it is required to determine some characteristics of the radiation field. This means that schematically the direct problem may be represented as follows: X+A+G+S→J. The inverse problems of atmospheric optics may be formulated in different ways. The classification of different inverse problems of the atmospheric optics is given in Table 10.2. It is assumed that the geometry of the medium is given for all these problems, as it is usually the case in practice. The first type of inverse problems is different remote methods of measurement. They can be represented by the scheme: J+A+G→X The measured characteristics of the radiation field and the given parameters of the interaction of radiation with the medium and the boundary conditions are used to determine different parameters of the physical state of the atmosphere and the underlying surface. The simplest example of a problem of this type (determination of 409

Theoretical Fundamentals of Atmospheric Optics Table 10.2 Classification of inverse problems of atmospheric optics [71] Ty p e

Given

To be determined

Comments

1

J, A, G

X

Remote methods of measuring physical parameters of medium

2

J, X, G

A

Determination of optical parameters of medium

3

J, X, A

G

Determination of boundary conditions

the content of the absorbing gas) was presented previously. The second type of inverse problems of atmospheric optics is directed to determining optical characteristics of the atmosphere. This type of inverse problems can be represented by the scheme: J+X+G→A. It should be noted that this approach is traditional in the laboratory investigations of the optical characteristics of atmospheric gases. His approach is also used when determining the optical properties of a real atmosphere. The third type of inverse problems of atmospheric optics is formulated in relation to the boundary conditions as follows: J+X+A→G. The best known example of this type is the problem of determination of the integral or spectral solar constant on the basis of the ground-based or balloon measurements of direct solar radiation.

10.2. Remote measurement methods Two types of methods are used for measuring the parameters of the atmosphere and the underlying surface: contact and remote measurements. Contact measurements are measurements of a parameter at a specific ‘point’ (limited volume) of the atmosphere or surface (limited area). In contact measurements a sensitive sensor is in direct contact with the investigated object. An example – measurement of the temperature of the atmosphere using a thermometer. Contact measurements are used for a large number of measurements of different parameters of the atmosphere and the surface. However, it is almost impossible to obtain by these 410

Radiation as a Source of Information on Optical and Physical Parameters

measurements the information on the state of the atmosphere in the regional or global scale. To a great degree, this relates to other planets. Therefore, remote measurements are very important in atmospheric sciences. In a general case, remote measurements are based on the recording of the characteristics of different fields – gravitational, electrical, magnetic, electromagnetic, acoustic. In these methods, the characteristics of the medium are measured at a distance from the investigated volume of the atmosphere or surface area. These distances can very large, for example, for satellite measurements or in examination of planets from the surface of the Earth. In this monograph, we examine remote methods based on recording the characteristics of the electromagnetic field (radiation). The processes of generation of radiation or its transformation depend on the optical and physical parameters of the medium. These dependencies are also the physical basis of the examined remote measurement methods. Figure 10.1 shows the diagram of remote measurements. The most important element of these measurements is the measuring device. The input of the device receives electromagnetic radiation. The devices measure different characteristics of the radiation field – angular, spectral, polarization – depending on time and often on the point in space. At the output of the device the signals are proportional to some functionals of the radiation field. For example, in spectral measurements we obtain information on the spectral dependence of radiation with a specific spectral resolution: I ∆ν (ν) =

∫ I (ν)ϕ(ν − ν′)d ν,

∆ν

(10.2.1)

where ∆ν is the resolved spectral range; ϕ(ν–ν') is the spectral slit function of the device which describes quantitatively the reaction (response) of the device to radiation with different frequency. The device also carries out angular and time averaging of intensity of radiation described by the appropriate slit functions. It is important to stress that for subsequent interpretation of measurements, i.e. obtaining important optical or physical parameters of the medium, it is necessary to know different characteristics of the device. These characteristics are usually examined in advance in a laboratory. To solve the inverse problems of atmospheric physics, including problems in remote sounding of the medium, as indicated by Table 10.2 and Fig.10.1, it is necessary not only to measure the specific 411

Theoretical Fundamentals of Atmospheric Optics

Radiation Apriori information Interaction parameters A

Device

Characteristics of device

Radiation functionals

Boundary conditions G Geometry of medium S Class of solution

Algorithm of processing measurement data

Estimates of solution, errors

Fig.10.1 General block diagram of remote measurements [71].

characteristic of radiation but also provide a certain amount of information which is referred to as a priori. The a priori information differs depending on the type of inverse problems. Thus, the remote methods of measurement of the parameters of the atmosphere and the underlying surface include not only the measuring device but also a specific amount of the a priori information and also the algorithm of retrieving the required parameter. Therefore, the accuracy of remote sounding by a medium depends not only on the accuracy of measurements of some characteristics of the radiation field but also on the accuracy of a priori information. Direct measurements of the device are only the initial stage of realization of remote measurements, they do not solve the given task of determination of the parameters of the medium. This is carried out in the algorithm of processing the measurement data (algorithm of interpretation) – a special system of calculation codes for a computer. This algorithm includes the a priori information on the model of radiation transfer, the parameters of interaction of radiation with the medium, the boundary conditions and the geometry of the medium. This information, included in the equation of radiation transfer, forms the physical–mathematical model of remote methods of measuring the parameters of the atmosphere and the underlying surface. As mentioned previously, this model should 412

Radiation as a Source of Information on Optical and Physical Parameters

also contain different characteristics of the measurement device – spectral and angular resolution, appropriate slit functions, different measurement errors, and so on. It may be shown that the majority of the inverse problems of atmospheric physics are reduced, from the mathematic viewpoint, to solving integral equations of a special type – Fredholm integral equation of the first kind [75]:

∫

y( x ) = K ( x, y) f ( y)dy, b

(10.2.2)

a

where y(x) is the known (measured) function; K(x,y) is the kernel of the integral equation; f(y) is the function to be determined. In this case y(x) is the characteristics of radiation; f(y) is the required parameters of the atmosphere or surface. The relationship (10.2.1) also belongs to this type of equation. Infact, if we want to obtain, from the measurements of the radiation intensity in finite spectral intervals I ∆ν (ν), the monochromatic intensity at the input of the device I(ν) at the known kernel of the equation K(x, y) = ϕ(ν–ν'), it is necessary to solve the equation of type (10.2.1). A simple example relating to atmospheric optics is the example presented previously for the optical thickness of the atmosphere. In a general case, for an inhomogeneous atmosphere the optical thickness is expressed as follows τ ( ν ) = ∫ k ( ν ,z ) ρ ( z ) dz

(10.2.3)

If we know the optical thickness, then to determine the vertical distribution of the density of the absorbing gas (with the given absorption coefficient K(x,y)) it is necessary to solve the Fredholm integral equation (10.2.2) A general special feature of these equations is their incorrectness in the classic sense (according to Hadamard) which leads in particular to the need to specify another type of a priori information – the class of the solution to be found. An exception is represented by the inverse problems of remote refractometry reduced to solving the integral Volterra equations of the first kind which are correct according to Hadamard. These special features of the solution of the inverse problems require the application of special algorithms of interpretation of the measured data. Like any other measurement methods, the remote measurements

413

Theoretical Fundamentals of Atmospheric Optics

are characterised by the errors (random and systematic) of the measurement of the sought parameter of the atmosphere or underlying surface. In addition to this, measurements in atmospheric physics, meteorology, climatology, etc (including remote measurements) are characterised by the spatial (horizontal and vertical) resolution, periodicity of measurements, and the rate of transfer of the results of measurements to users. Since remote measurements represent a set of the measuring device (with its measurement errors), the specific amount of a priori information and the interpretation algorithm, all these components determine the accuracy characteristics of remote measurements.

10.3. Classifications of remote measurement methods Because of the existence of a large number of remote measurement methods in atmospheric optics it is convenient to consider their general classification. The remote measurement methods of the parameters of the environment are classified according to different criteria: – the type of radiation used (nature of radiation, source of radiation); – the main processes of interaction of radiation with the investigated medium; – illumination conditions (the time of day); – the spectral range; – the parameter to be determined; – the carrier used. Firstly, the remote measurement methods are subdivided into passive and active (depending on the nature of radiation used). The passive methods, using the measurements of the characteristics of natural radiation fields, include: 1) Methods of atmospheric radiation (equilibrium and nonequilibrium); 2) Methods of scattered radiation (solar and reflected from the moon); 3) Methods of attenuation and absorption (transmittance) mainly of solar radiation, but also the radiation of the moon and stars; 4) Refraction methods; 5) Methods of reflected radiation. 6) In many cases, the second and fifth passive methods are combined into a single method – the method of scattered and reflected radiation.

414

Radiation as a Source of Information on Optical and Physical Parameters

The active methods of sounding, using artificial sources of electromagnetic radiation, are: – lidar sounding; – radar sounding; – the refraction method; – the extinction and absorption method. The classification of the remote measurement methods on the basis of the main processes of interaction of radiation with the investigated medium is close to that described previously. This classification specifies: – scattering methods (various types – molecular (Rayleigh), aerosol, Raman, etc); – extinction (absorption) methods; – atmospheric radiation methods; – refraction methods, etc. In this classification, the methods of absorptions, scattering and refraction are used in both passive and active measurement methods. According to the illumination conditions (the time of day) the remote methods can subdivided into: 1) day methods (above the illuminated side of the planet); 2) night methods; 3) methods used in the terminator region (the region of transition from the day to night side of the planet). The latter methods, especially in goundbased measurements, are often referred to as the methods of twilight sounding. The first and third passive methods are associated with the application of solar radiation as a source of information on the medium state and are used for the day side of the planet. At night, the remote methods can also be based on the measurement of radiation of stars and of the solar radiation reflected from the Moon, and also on the measurement of various glows of the atmosphere. In principle, the active sounding methods can also be used at any time of day. However, the presence during the day of a high level of reflected and scattered solar radiation complicates the application of, for example, lidar methods in the visible and near infra-red ranges of the spectrum above the day side of the planet. From the viewpoint of the carrier used, the remote methods are subdivided into ground-based, aircraft, balloon, rocket and space. According to the geometry of measurements, the cosmic methods can be sub-divided into the methods of nadir and limb (on the horizon of the planet) sounding. Figure 10.2 shows different types

415

Theoretical Fundamentals of Atmospheric Optics

ra

n

io

ar

at ol

di

ra

Nadir and oblique measurements of thermal radiation and scattered solar radiation Tr

Satellite orbit Conditional Li upper mb boundary s t h e r m ca tte al of re d atmosphere s

me

as

ur

em

en

ts o an f di d at io n

Solar radiation

an m mit et ta ho nc d e

Satellite

Fig.10.2 Different types of geometry and methods of space measurements.

of geometry and different methods of space measurements. In nadir geometry of measurements (or close to those – oblique), the outgoing radiation is recorded in directions in the vicinity of the local vertical. The majority of currently available satellite devices carries out angular scanning in the vicinity of the nadir (in the majority of cases normal to the plane of the orbit) so that the horizontal fields of the investigated characteristics can be determined. The identical result may be obtained using special receivers – linear or matrix type. The range of scanning angles, the angular aperture of the devices, the type of scanning and the height of the space carrier determine the spatial range of measurements and the horizontal resolution of remote measurements. The remote methods of measuring the parameters of the atmosphere and underlying surface can be classified by the parameter to be determined. In this classification there are remote methods for determination of: – temperature, density and pressure of the atmosphere; – characteristics of the clouds – amount, the height of upper and lower boundary (vertical structure), temperature of the upper boundary (UB), water content, phase composition, and microphysics of clouds; – intensity of precipitation; – the content of absorbing gases – water vapour, ozone and other trace gase (TG); – wind fields;

416

Radiation as a Source of Information on Optical and Physical Parameters

– aerosol characteristics (optical and microphysical); – properties of the underlying surface – temperature, moisture content and the optical characteristics of the underlying surfaces (reflectivity and emissivity).

10.4. Remote methods of measurement based on measurements of attenuation (absorption) of radiation These methods were already discussed when we considered the concepts of the inverse problems of the theory of transfer and atmospheric physics. In the majority of cases, these methods use the radiation of the sun and they are applicable during the day time, although a large number of examples in recent years have been associated with the utilization of radiation of stars and of the solar radiation reflected from the moon. The two latter cases enable this remote method to be used at night. The considered methods are most efficient in studying the characteristics of the gas and aerosol atmospheric state, and also in a number of cases of clouds. In addition to this, using the temperature dependence of molecular absorption, it is possible to determine the temperature of the atmosphere. Selecting different spectral ranges and spectral intervals from the ultraviolet range to the radio range in measurements, for example of solar radiation, we can obtain information on the content of tens of different gases in the atmosphere. These experiments have been conducted from the surface of the Earth for a very long time. It is interesting to note that these remote methods have been used to detect the presence of ozone and many other gases in the Earth’s atmosphere. The discussed remote methods are often referred to as the transmittance methods because the information on the physical parameters of the physical state is extracted in fact from the transmittance functions of the atmosphere (transmissivity of the atmosphere). Taking into account different factors attenuating the radiation, the optical thickness may be represented in the form: τ(ν, s1 , s2 ) =

∫ ∑ k (ν, s) ρ ( s)ds + τ

s2 N

s1 i =1

i

i

R (ν , s1 , s2 ) +

τ a (v, s1 , s2 ),

(10.4.1)

where k i (ν,s) is the coefficient of the absorption of the i-th gas component; ρ i (s) is its density; τ R (ν,s 1 ,s 2 ) and τ a (ν,s 1 ,s 2 ) are the optical thicknesses of Rayleigh and aerosol extinction. It should be mentioned that equation (10.4.1) was written for the case of the monochromatic optical thickness. In actual experiments it is 417

Theoretical Fundamentals of Atmospheric Optics

necessary to take into account the finite spectral resolution of the devices. Thus, the examination carried out here is idealized because the measurements of strictly monochromatic radiation are not possible. However, there are devices with super high spectral resolution (for example, heterodyne spectrometers) for which the study of the monochromatic case is of practical interest. Equation (10.4.1) shows that the radiation extinction in a general case is determined by different physical reasons – true absorption by atmospheric gases, extinction as a result of molecular (Rayleigh) scattering, the absorption and scattering by atmospheric aerosols. Using different spectral dependences of different extinction mechanisms, we can define different components of this extinction. Depending on the spectral range of measurements, the characteristics of the device and the aims of interpretation, equation (10.4.1) represents the basis of different remote methods: – determination of the characteristics of the gas composition of the atmosphere; – determination of the temperature of the atmosphere; – measurement of the velocity of wind; – determination of the density of atmosphere; – measurements of different characteristics of atmospheric aerosols. We consider different remote methods of measurement assuming for simplicity it is possible by some means to separate different components of the radiation extinction in equation (10.4.1).

Determination of the characteristics of the gas composition of the atmosphere by the transmittance method Firstly, it should be mentioned that in the ground-based experiments there is a problem of determination of the intensity of the extraatmospheric solar radiation I 0 (ν). This intensity is determined using the long or short Bouguer methods. For the model of a planeparallel horizontally homogeneous atmosphere we can write the equation for the optical thickness:

∫

∞

1 k (ν, z ) ρ( z )dz. τ ρ (ν ) = cos θ 0

(10.4.2)

The success of solving the inverse problem – determination of the vertical profile of the density of the absorbing gas ρ(z) from equation (10.4.2) – depends on the behaviour of the kernel of the 418

Radiation as a Source of Information on Optical and Physical Parameters

equation (the absorption coefficient k(ν,z) in this case) in relation to altitude at different frequency. Let us assume that, for example, the absorption coefficient does not depend on altitude. We can therefore carry out evident transformations:

∫

∫

k (ν ) k (ν ) τρ ( ν ) = ρ( z )dz = u , u = ρ( z ) dz. cos θ 0 cos θ 0 ∞

∞

(10.4.3)

Equations (10.4.3) show that in the examined case we can not obtain information on the vertical profile of the density of the absorbing gas but can obtain information only on its integral content in the entire thickness of the atmosphere. It should be mentioned that the determination of the value of the total content u of different gases is often of considerable practical interest. For example, ground-based measurements of the absorption of ultraviolet solar radiation enable us to determine the integral (total) content of ozone with a high accuracy (1–3%). This is associated with the fact that the absorption coefficient of ozone in the ultraviolet range of the spectrum does not depend on the pressure of the atmosphere and depends only slightly on the temperature of the atmosphere. Groundbased stations for measuring the total ozone content operate in many countries of the world. For example, in CIS countries measurements of this type are now carried out at ~40 ozone measuring stations. In addition to this there is a network of stations carrying out surface spectroscopic measurements of the total content of CH 4 , CO, H 2 O, CO 2 , N 2 O in the infrared range of the spectrum. In the infrared range of the spectrum, the dependencies of the molecular absorption of the atmospheric gases greatly differ in the majority of cases. As shown previously (see Chapter 4), the molecular coefficient of absorption in the spectral line k(ν,z)=k(ν,p(z), T(z)). The relationships for the Lorentz line (derived from analysis of the equations (4.5.20) and (4.5.24)), presented in Chapter 4, show that in the centre and the wing of the line the dependences of the absorption coefficient on pressure and, consequently, altitude in the atmosphere, greatly differ. The absorption coefficient in the wing of the line is directly proportional to pressure, whereas in the centre of the line it is inversely proportional. Thus, the contributions to the optical thickness of the molecular absorption of different layers of the atmosphere in different sections of the Lorentz line greatly differ and it is intuitively clear that solution of the integral equation (10.4.2) may provide information in particular on the vertical profile of the 419

Theoretical Fundamentals of Atmospheric Optics

density of the absorbing gas. This information can be obtained from the measurements with high spectral resolution enabling us to scan different spectral lines. This is realized in the microwave range of the spectrum. At middle spectral resolution the majority of the interpretation methods are based on the application of the model (mean) vertical profile of the content of the absorbing gas and the problem of determination of the total gas content is solved. We consider another example of the application of the measurements of atmospheric transmittance. Figure 10.3 shows the geometry of space measurements of solar radiation. In movement of a satellite on the orbit, the sun rises or sets beyond the horizon of the planet in relation to the satellite. At these moments, the spectral devices installed on the satellite, can be used for measurements of solar radiation both during passage of the radiation through the atmosphere and outside it. In this case we can determine the transmittance function of the atmosphere on slant paths, i.e. the ratio I(ν)/I 0 (ν). These methods of sounding the atmosphere are often referred to as twilight methods because they are carried out during the ‘darkening’ of radiation by the Earth (the Sun in the present case). The Bouguer law for the considered case can be written in the form  ∞  I (ν, h0 ) = I 0 (ν) exp  −2 w( z , h0 )k (ν, z )ρ( z )dz  ,  h   0 

∫

(10.4.4)

where h 0 is the tangent height, i.e. the minimum distance of the trajectory of propagation of a light ray from the surface of the Earth (Fig.10.3); w(z,h 0 ) is the Jacobian of transition from coordinate f along the path of propagation of radiation to altitude z. The multiplier 2 forms as a result of division of the path into two identical integration sections which holds for the model of the spherically homogeneous (layer-stratified) atmosphere. It should be mentioned that the measurements of solar radiation may be carried out as a function of the frequency and trajectory of propagation of radiation characterised by, for example, tangent height h 0 . It is also important to mention that in space experiments (in opposite to the ground-based measurement method when there is a problem of determination of the extra-atmospheric value of solar radiation I 0 (ν), solved by special methods) we can measure directly the extra-atmospheric radiation at high values of tangent

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Sun Device

Fig.10.3. Geometry of cosmic measurements of solar radiation.

height h 0 . Passing in the monochromatic case to the optical density of molecular absorption we obtain

∫

∞

τ(ν, h0 ) = 2 w( z , h0 )k (ν, z )ρ( z )dz.

(10.4.5)

h0

Here the kernel of the integral equation is the product w(z,h 0 )k(ν,z), which is assumed to be known, and the function to be determined is the density of the absorbing gas ρ(z). Equation (10.4.5) is the Volterra equation of the first type since the integration limit is variable. Formally, we can solve this equation analytically differentiating the optical density in respect of tangent height h 0

ρ(h0 ) = −

d τ(ν, h0 ) 1 . dh0 2 w(h0 , h0 )k ′(ν, h0 )

(10.4.6)

The problems of a stable numerical solution of equation (10.4.5) remain because the numerical differentiation of the measured function also belongs to ill-posed (in the classic sense) problems. It should be mentioned that our examination was carried out for the intensity of radiation. Just as devices cannot measure monochromatic radiation, and measure the radiation energy in finite spectral range (this is taken into account by integration over the spectrum taking into account the slit function of the device), they measure the radiation energy infinite solid angles. This is taken into account by integrating the intensity of radiation in finite solid angles taking the angular sensitivity of the devices into account. In the discussed method, measurements at different tangent heights give a possibility to obtain information on the vertical structure, and the spectral extinction dependences make it possible to separate contributions to the extinction of different components, in particular, the absorption of different gases.

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Altitude, km

The possibility of extracting actual information on the vertical profile of the density of the absorbing gas is associated with the behaviour of the kernel of the equation (10.4.5) as a function of altitude at different tangent heights. Figure 10.4 gives the kernels of the integral equation (10.4.5) for the mixing ratio of ozone q(z) in measurements of solar radiation in the infra-red region of the spectrum at a wave length of 285 nm in the region of the Hartley– Huggins ozone absorption band at different tangent heights in the atmosphere. It is seenthat at different tangent heights, the kernels are placed at different atmospheric altitudes. This behaviour of the kernels is also a physical basis of the possibility of determining by space measurements atmospheric transmittance the information on the vertical profile of the mixing ratio or the density of the absorbing gas. The character of the dependence of the kernels of equation (10.4.5) on altitude in the atmosphere is determined by the specific features of the geometry of space measurements, in particular by the fact that at the given tangent height h 0 solar radiation is not absorbed by the layers situated below this tangent height. The spaxe scheme of measurements of the characteristics of the gas composition in the atmosphere was used in the last couple of decades in many experiments of different ranges of the spectrum.

Fig.10.4. Kernels of integral equations (14.4.5) for the ozone mixing ratio q(z) = ρ O3/ ρa in measurements of solar radiation in the region of the Hartley–Huggins ozone absorption band (λ=285 nm) at different tangent heights in atmosphere. 422

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The devices with high spectral resolution and the selectivity of the spectra of molecular absorption enables us to study the content of many atmospheric gases in the upper troposphere, stratosphere and mesosphere. For example, in experiments with the infrared interferometer ‘ATMOS’ with high spectral resolution, the vertical profiles of more than 30 atmospheric gases were measured simultaneously. The results of measurements of the Sun radiation in the ultraviolet, visible and near-infrared ranges of the spectrum by spectrometers SAGE-I, SAGE-II, POEM, OZON-MIR and others were used to determine the content of ozone, NO 2, H 2 O and aerosol extinction. Figure 10.5 shows an example of the retriveal of the vertical profile of the ozone content from the results of measurements with OZON–MIR spectrometer (long-term orbital station MIR). The same graph gives the ozone profile determined using a different remote method (limb method) based measuring the thermal radiation of the horizon of the planet (EMS equipment). The majority of space experiments of this type use the measurements of the radiation of the sun. However, in this case the number of these measurements per day is not large. Recently, similar measurements have been taken using the radiation of different stars. In this case it is possible to increase greatly (by an order of magnitude or more) a number of measurements per day. It is also possible to increase greatly the spatial coverage of different regions of the Earth by measurements

Pressure, mbar

Orbit 2575, 01/02/97, 7°NL, 63°WL

Ozone volume mixing ratio, ppm V

423

Fig.10.5. Example of restoration of the vertical ozone profiles from measurements with OZON–MIR spectrometer (1) and limb thermal radiation measurements (2) of the horizon of the planet [56] (equipment MLS).

Altitude, km

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Ozone concentration, cm 3 ·10 12 Fig.10.6. Example of retrieval of the vertical ozone profile from measurements of star radiation (experiments with UVISI equipment on board USA MSX satellite). 1) lidar sounding; 2) occultation experiment.”

in comparison with the use of solar radiation. Figure 10.6 gives an example of the retrieval of the vertical profile of the ozone content from the measurements of radiation of stars (experiments with equipment UVISI on board of the American satellite MSX). To verify the results of remote satellite measurements, Fig.10.6 also gives the profile of the ozone content determined using ground-based lidar sounding.

Determination of the vertical profile of temperature by the transmittance method We now return to the expression for the optical density of molecular absorption for the space geometry of measurements – equation (10.4.5). Previously it was assumed that the density of the absorbing gases is the required function. However, we assume that this function is known. This assumption in the Earth’s atmosphere is fulfilled with high accuracy and in a wide range of latitudes for various atmospheric gases such as oxygen and carbon dioxide. In this case we can formulate the inverse problem in relation to the vertical profile of the temperature of the atmosphere T(p). Actually, as mentioned previously, the intensity of the spectral lines (and also 424

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half-widths but in most cases to a lesser extent) depends on temperature (Chapter 4). For example, for the intensity of vibrational–rotational lines of linear molecules we can write to a first approximation:

 T  1 1  S (T ) = S0   exp  k B E ′′  −   ,  T0   T0 T   

(10.4.7)

where S 0 is the intensity of the line at reference temperature T 0 ; E'' is the energy of the low vibration level of the quantum transition of the molecules; k B is the Boltzmann constant. As shown by the relationship (10.4.7), depending on the values of energy E'' the dependence of the intensity of the line on temperature of the atmosphere differs. For example, for a CO 2 absorption line situated close to the frequency of 668.60867 cm –1 , the value of E'' is equal to 464.1717 cm –1. Simple calculation shows that for this line the derivative dS(T)/dT at T 0 = 296 K is approximately equal to 0.01S 0 [K –1 ]. Thus, when the temperature of the atmosphere varies by 1 K, the intensity of the line, and consequently, optical thickness τ(ν) change by 1% which may be recorded by space devices. This means that in the examined case, equation (10.4.5) is a non-linear integral Volterra equation of the first kind in relation to the vertical profile of the temperature of the atmosphere.

Determination of the vertical profile of the density of the atmosphere by the transmittance method The density of the atmosphere can be determined on the basis of different physical principles. We shall therefore discuss two remote methods: – the method of molecular absorption; – the method of extinction as a result of Rayleigh scattering. The first method is based on relationship (10.4.5). It is used to determine, for example, for the cosmic measurement scheme, the vertical profile of the density of the i-th gas component of the atmosphere. The determination of the density of gas components such as O 2 or CO 2 whose mixing ratios in the Earth’s atmosphere are known and are constant in time and in space, enables us to determine the total density of the atmosphere on the basis of the relationship

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ρ a = ρ i / q i,

(10.4.8)

where ρ a is the total density of the atmosphere. Substituting the relationship ρ i (z) = ρ a (z)q i (z) into equation (10.4.5) gives the explicit form of the integral equation for determining the total density of the atmosphere

∫

∞

τ(ν, h0 ) = 2 w( z, h0 )k (ν, z)qi ( z)ρa ( z)dz,

(10.4.9)

0

where K(z,h 0 ,ν)=w(z,h 0 ) k(ν,z)q i (z) is the kernel of the integral equation. To illustrate the second approach we assume for simplicity that the only component of extinction of radiation is the optical thickness of Rayleigh (molecular) scattering. The data presented previously (Chapter 5) show that the Rayleigh scattering coefficient depends on concentration of air molecules. Consequently, we can formulate the appropriate integral equation for the density of air.

Determination of the characteristics of atmospheric aerosols by the transmittance method Different inverse problems in relation to the characteristics of the atmospheric aerosols have also been formulated for the transmittance methods. To simplify considerations, it is assumed that the extinction of radiation takes place only as a result of scattering and absorption on atmospheric aerosols. Therefore, the optical density in relationship (10.4.1) is determined by the extinction of radiation on aerosols. For the case of extinction on spherical homogeneous particles for polydispersed aerosols particles we can write the following expression for the optical thickness of aerosol extinction τ a (λ) (5.3.2):

∫

∞

τ a (λ ) = sec θ α a ( z )dz = 0

= sec θ

∫∫ N ∞∞

a ( z )πr

2

Qe (r , m, λ, z )na (r , z )dzdr,

(10.4.10)

0 0

where Q e (r,m,λ,z) is the extinction factor of particles with radius r with the complex refractive index (CRI) m; n a (r,z) is the size 426

Radiation as a Source of Information on Optical and Physical Parameters

distribution function of the particles. It should be mentioned that in equation (10.4.10) we took into account the dependence of Q e and n a on altitude in the atmosphere. For the case of an aerosol– homogeneous atmosphere, i.e. for Q e and n a independent of height, we can formulate the following integral relationship in relation to the normalized distribution function f a (r):

∫

∞

τ a (λ ) = sec θN 0 πr 2Qe (r , m, λ ) fa (r )dr,

(10.4.11)

0

where N 0 is the total number of the aerosol particles. Measuring the spectral dependence of the optical density of aerosol extinction, we can determine f a(r) [29,84]. The possibilities of the examined methods are associated with the behaviour of the kernel of this equation as a function of the particle radii at different wavelengths. Figure 10.7 shows the kernel of this equation for the volume distribution function of the particles for the case of particles with the complex refractive index m =1.60–0.02i. Equation (10.4.11) shows that to calculate the kernels of the equation it is necessary to know the physical–chemical characteristics of atmospheric aerosol – its complex refractive index m. This situation is realised, for example, for water aerosol, i.e. for clouds and precipitation. In a general case, the problem of remote determination of the aerosol characteristics should be formulated as a problem of simultaneous determination of f(r) and m.

10.5. Remote methods using measurements of atmospheric radiation A large group of inverse problems of atmospheric optics for atmospheric radiation has been formed. These problems should be sub-divided into the methods of thermal radiation, the infrared nonequilibrium radiation and methods using the measurements of glow of the atmosphere. These methods are based on the integral form of the equation of radiation transfer (Chapter 3).

Thermal radiation methods For the outgoing thermal radiation for a plane–parallel horizontal homogeneous atmosphere with nadir geometry of measurements we

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Normalised kernels

λ = 0.77µm λ = 0.347µm λ = 1.68 µm

Particle radius, µm Fig.10.7. The kernels of equation (10.4.11) for the volume distribution function of the particles for the case of particles with the complex refractive index m = 1.60–0.02i [117] for different wave lengths.

can write:

∫

 ∞  I (ν, θ) = Bν (Ts )exp  − kν ( z )ρ( z )dz  +    0 

∫

∫

 ∞  + Bν (T ( z))kν ( z)ρ( z )exp  − kν ( z′)ρ( z′)dz′  dz.   0  z  ∞

(10.5.1)

When writing equation (10.5.1) we have used the assumptions on the establishment of local thermodynamic equilibrium, on the negligible effects of scattering of thermal radiation and the absolutely black underlying surface. Equation (10.5.1) is a physical– mathematical basis for formulating different inverse problems of the thermal region of the spectrum for space and ground-based measurement schemes. In the later case, in infrared spectral range, the integrated term describing the contribution of the underlying surface is absent. For the microwave range, this integrated term takes into account 428

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the relic cosmic radiation falling on the upper boundary of the atmosphere. We discuss examples of these inverse problems. The integral form of the equation of transfer of thermal radiation (equation 10.5.1) shows that thermal radiation depends on the temperature and characteristics of the content of the absorbing and emitting components (for example, different gases). In a general case we can write I(ν,0) = I[T 0 , T(z), q i (z), a, b, c,...],

(10.5.2)

where a, b, c, etc, characterise the optical properties of the atmosphere and the underlying surface. As already mentioned, in remote measurement methods it is assumed that these parameters are known. However, in this case equation (10.5.2) shows that thermal radiation is determined by the entire set of the functions describing the physical state of the atmosphere – vertical profiles of temperature, and the mixing ratios of different gases, in a general case by the characteristics of clouds and aerosols, etc. Even if we can measure the spectral and angular dependences of thermal radiation, it is difficult to hope to extract from these measurements all required the information on the parameters of the atmosphere and the underlying surface. Here ‘help’ comes again from the spectral selectivity of the optical properties of the atmosphere. In the entire thermal range of the spectrum from 3–4 µm to radio waves there are ‘spectrally localized’ absorption bands of different gases and ‘transmittance windows’ – the range in which atmospheric extinction is relatively small. Although the absorption bands of different gases overlap and there are no absolutely ‘clean’ transmittance windows, this property does make it possible to a certain degree to ‘separate the variables’ in the relationship (10.5.2).

Determination of the temperature of the underlying surface If it is assumed that the atmosphere does not absorb radiation, then equation (10.5.1) retains only the integrated term describing the radiation of the underlying surface for the space measurement scheme. In real conditions, the absorption (and radiation) of the atmosphere is also detected in the transmittance windows. However, it is important that, for example, in the 8–12 µm transmittance window, the outgoing thermal radiation depends mainly on the temperature of the surface or the characteristics of clouds (if they are located in the field of view of the satellite device). When solving the given problem, in addition to the previously 429

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mentioned range 8–12 µm (or narrower spectral ranges in this region) we utilize the transmittance window in the near infrared range of the range (3.7.µm) and also the optically transparent intervals in the microwave range. The accuracy of determination of the temperature of the underlying surface depends on the accuracy of taking into account the effect of the atmosphere, the accurate assignment (or independent determination) of the emitting properties of the surfaces, and the effect of clouds (in the infrared region of the spectrum). At present, this remote method makes it possible to measure the temperature of the surfaces of oceans with the accuracy of 0.5–1.0 K.

Determination of the vertical profile of temperature (nadir measurement geometry) It is now assumed that we have determined the temperature of the underlying surface. It is also assumed that the measurements of the spectral angular dependence of the outgoing thermal radiation are performed in the absorption bands of atmospheric gases whose concentrations are known and remain constant with time, and in space. Such gases in the atmosphere of the Earth are CO 2 and O 2 . Consequently, equation (10.5.2) retains one unknown function – the vertical temperature profile. Methods have been developed and applied for determining the vertical profile of temperature from measurements of the spectral dependence of outgoing radiation in the infrared absorption bands of carbon dioxide at 15 and 4.3 µm and in the absorption band of oxygen in the microwave range at 0.5 cm. A separate line of absorption of oxygen at 0.25 cm can also be used for this. The possibility of remote determination of the vertical temperature profiles is connected with the fact that the outgoing radiation in the spectral ranges with different optical densities is generated by different altitude layers of the atmosphere. This mechanism is shown in Fig. 10.8 which shows the so-called weight functions of the investigated inverse problem characterising the regions of formation of the outgoing thermal radiation in different spectral intervals of 4.3 and 15 µm CO 2 absorption bands. The weight functions are the derivatives of the transmittance functions of the atmosphere in the integral form of the transfer equation which ‘weigh’ the altitude distribution of the Planck function or the temperature in the microwave range. For the microwave range they are very close to the kernels of the integral

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Pressure, mbar

Channel

Weight functions Fig.10.8. Weight functions characterising the regions of formation of outgoing thermal radiation in different spectral intervals of the absorption bands of CO 2 at wavelengths of 4.3 and 15 µm [105].

equation of variational derivatives, if the temperature dependence of the transmittance function is not taken into account. For the infrared range the kernels differ from the weight functions by the multiplier – the derivative of the Planck function in respect of temperature. The weight functions together with the kernels of the integral equations and variational derivatives are often used for analysis and illustration of the possibilities of remote measurements.

Determination of vertical profile of the temperature (limb geometry of measurements) The outgoing radiation can be measured directing the device, mounted on board a satellite, to the horizon of the planet. In this case, we deal with the limb geometry of measurements or the measurement of the radiation of the planet horizon. In this 431

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measurement scheme the device should have a sufficiently high angular resolution or, in other words, it should be capable of recording the outgoing radiation in a narrow range of tangent heights (Fig.10.2). The transfer equation for the considered geometry of the measurements can be written in the form:

∫

∞

I (ν, h0 ) = Bν (T ( z )) w( z , h0 ) × 0

 ∞  ∂  exp  − kν ( z′)ρ( z′) w( z′, h0 )dz′  +   ∂z   z  

∫

(10.5.3)

∞  z  + exp  − kν ( z′)ρ( z′) w( z′, h0 ) dz′ − kν ( z′)ρ( z′) w( z′, h0 ) dz′   dz ,  h  h0  0 

∫

∫

where w(z,h 0 ) is the Jacobian of transition from integration along the path of formation of radiation to the integration with respect to the vertical coordinate. Equation (10.5.3) holds for the model of a spherical layered-homogeneous atmosphere which assumes that the temperature of the atmosphere depends only on the vertical coordinate. In other words, we ignore the presence of horizontal temperature gradients on the path of formation of outgoing radiation. Different exponents in equation (10.5.3) take into account the contributions of different elements of the path of formation of radiation to the outgoing radiation of the horizon. Figure 10.9 shows the behaviour of another characteristic of the formation of outgoing radiation, namely the contribution of the layers of the atmosphere to outgoing radiation which together with the kernels, the weight functions and the variation derivatives are also used for analysis of the problems of remote sounding of the atmosphere. Figure 10.9 shows the contributions to the radiation of layers at different measurement tangent heights for the problem described by the equation (10.5.3) for a spectral interval in the 15 µm CO 2 band. In the problem of thermal sounding discussed here, the contributions of the individual layers of the atmosphere to the outgoing radiation are equal to the product of the Planck function by the difference of the transmittance functions at the boundaries of the layer. They characterise the fraction introduced by the layer of the atmosphere to the outgoing radiation. It is important to note that these weight functions are equal to zero below the measurement tangent height because of the fact that the subjacent layers (at a fixed tangent height) have no effect on the

432

Height, km

Radiation as a Source of Information on Optical and Physical Parameters

Relative contribution to radiation Fig.10.9. Contribution of atmospheric layers at different tangent heights h 0 to the outgoing radiation for the spectral interval in the CO 2 band at 15 µm [35].

formation of outgoing thermal radiation (in the absence of scattering and under LTE conditions). Therefore, the type of functions characterising the contributions to the outgoing radiation of different layers of the atmosphere is slightly different in comparison with the nadir geometry of measurements. These functions show a rapid decrease to zero for the appropriate tangent heights. It is clear that this special feature should result in higher vertical resolution of the discussed remote method of determination of temperature profile in comparison with nadir geometry. It should be stressed that for the limb geometry of measurements in solving the inverse problems we use the measurements of the angular dependence of outgoing radiation or, which is the same, dependences of the intensity of radiation on the measurement tangent height. This means that to solve this problem it is sufficient to measure the outgoing radiation in one spectral range of the CO 2 or O 2 absorption bands. In a general case, we can use both angular and spectral dependences. Then we can 433

Theoretical Fundamentals of Atmospheric Optics

determine in an independent manner the vertical profile of pressure and avoid the assumption on the spherical layer-stratified model of the atmosphere. In this case we can formulate a more complicated inverse problem in respect of the determination of the temperature of the atmosphere as the function of altitudes and horizontal coordinate. The considered schemes of satellite experiments (nadir and tangential) have been used many times in a large number of space experiments. Nadir thermal sounding of the atmosphere is carried out at present to obtain global information on the three–dimentional temperature field at altitudes from 0 to 40–50 km. The most informative systems of temperature sounding use the measurements of outgoing radiation in all spectral ranges suitable for solving the examined problem, – the absorption bands of carbon dioxide at 4.3 and 15 µm and the microwave band of oxygen. This integrated scheme is especially effective is sounding the cloudy atmosphere because of the fact that microwave radiation is only slightly affected by the clouds. Limb sounding is carried out for scientific research problems and makes it possible to determine the altitude profile of the temperature in the altitude range 10–120 km. This method was used for the first time to obtain climatological information on the temperature condition of the atmosphere of the Earth for the middle atmosphere. Current errors of temperature sounding are ~1–2 K.

Determination of the characteristics of the gas composition of the atmosphere Knowing (determining) the vertical profile of temperature and using measurements in the absorption bands (radiation bands) of other gas components, we can obtain information on the content of, for example, water vapour, ozone and other gases. When solving these problems, we can used both nadir and limb geometry of measurements. Attention should be given to the fact that the required characteristic, for example, the vertical distribution of the densities of absorbing gases, are located in the index of the exponent of the transmittance function and under two integrals in respect of the spatial variable. Evidently, the smoothing effect of these operators should complicate obtaining the information on these profiles. In addition to this, it may be shown that for the nadir scheme of measurements in the conditions of the isothermal atmosphere and the absolutely black underlying surface the

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measurements of outgoing thermal radiation do not contain any information on the composition of the atmosphere. This conclusion follows from the definition of the absolutely black radiation as the radiation of an isothermal cavity. Figure 10.10 shows the kernels of the integral equation for determining the vertical profile of the content of water vapour obtained after linearization of equation (10.5.1) for the nadir scheme of measurements:

∫

∞

δI (λ, θ) = K (ν, z , θ)δρ( z )dz.

(10.5.4)

0

here δI(λ,θ) and δρ(z) are the variations of the intensity of outgoing radiation and of the content of the water vapour in relation to the mean values. The kernels are given for different frequencies in the contour of the spectral line of H 2 O in the infrared range of the spectrum at different shifts from the centre of the line (Fig.10.10). The figure shows the specific behaviour of the kernels of the discussed inverse problem for the infrared range of the spectrum – tendency of the kernel to zero in the vicinity of the Earth’s surface. This special feature is characteristic for the absolutely black underlying surface in the absence of a sudden change of temperature in the vicinity of the surface. For the limb geometry of measurements, the behaviour of the kernel of the appropriate integral equation is similar to that in Fig.10.9. This method, as the method used previously for temperature, is characterised by a higher vertical resolution (this is its significant advantage) in comparison with the nadir method. It should be mentioned that the methods of limb sounding have another advantage. Because of the considerably longer (in comparison with nadir measurements) paths of formation of outgoing radiation (tens of times), the limb measurement geometry provides information on the temperature and gas composition of the atmosphere up to considerably higher altitudes. At present, satellite systems for nadir sounding in the infrared and microwave ranges of the spectrum are used in the determination of the total content of water vapour and ozone. They also provide some information on the content of water vapour in individual layers of the atmosphere. Satellite sounding with limb measurement geometry is used to obtain vertical profiles of many gas components of the atmosphere (H 2 O, O 3 , CH 4 , etc.). The errors of determination of the characteristics of the content of the different 435

Pressure, mbar

Theoretical Fundamentals of Atmospheric Optics

Fig.10.10. Kernels of the integral equation for determining the vertical profile of the content of water vapour (nadir measurement scheme) for different frequencies in the contour of the spectral line of H 2O for different shifts from the centre of the line in fractions of its half widths α 0 . 1 – α 0/10; 2 – α 0/5; 3 – α 0/2; 4 – α 0.

gases depend on the given geometry or measurements and the type of gas. For total contents of the water vapour and the ozone for the nadir geometry of measurements these errors equal usually 5– 10 %. The vertical profiles of the content of different gases are determined with errors of 10–30%.

Determination of other parameters of the atmosphere and surface The measurements of thermal radiation are used to determine not only the previously mentioned parameters of the atmosphere and underlying surface – the temperature of the atmosphere and the surface, the characteristics of the gas composition, but also to determine the characteristics of the cloud cover (for example, the 436

Radiation as a Source of Information on Optical and Physical Parameters

water content of clouds) the intensity of precipitation, the properties of the underlying surface (emissivity, moisture content of soil) and the characteristics of atmospheric aerosols. One of the simplest but relatively important parameters of the cloud field – the cloud amount – is determined using devices with high horizontal resolution by analysis of the spatial pattern of the brightness temperature of radiation in different transmittance windows of the atmosphere. The clouds usually located in the troposphere have a considerable lower brightness temperature of the radiation than the temperature of the underlying surface and can be easily seen on its background. The method has been used for a long time to study different characteristics of the cloud cover of our planet – amount and altitude of clouds, their type. These investigations were used for developing the climatology of the cloud cover of the Earth. The importance of these investigations is associated with the fact that, as mentioned previously, the clouds are one of the most important parameters determining the climate of our planet.

Infrared sounding of non-equilibrium atmosphere At present, inverse problems for the non-equilibrium atmosphere become more and more important. Conventional and inverse problems of the thermal range of the spectrum are based on an important assumption on local thermodynamic equilibrium (LTE). As shown previously (Chapter 7), this assumption is however not fulfilled for the upper layers of the atmosphere. Recently, special attention has been given to the development of remote methods of sounding the non-equilibrium atmosphere for the limb measurement geometry – measurements of the radiation of the horizon of the Earth. The inverse problems of the non-equilibrium atmosphere are characterised by a number of special features: 1. The number of parameters, characterising the physical state of the atmosphere, greatly increases because the state of the atmosphere is no longer described by a single kinetic temperature and the total concentration of absorbing molecules. Since the Boltzmann law is not fulfilled, this state is additionally described by the population of the appropriate levels of the molecules and vibrational (electronic, rotational, etc) temperatures. The number of these parameters corresponds to the number of states of the internal energy of the molecules, and the transitions between these states form the field of outgoing radiation of the atmosphere. For example,

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for one of the formulations of the inverse problem of determination of the parameters of the non-equilibrium atmosphere from the measured infrared radiation in the absorption bands of CO 2 and O 3 the required values are 40 vertical profiles of the kinetic and vibrational temperatures, the content of O 3 and CO 2 and pressure. In the LTE conditions, this problem is reduced to finding only four profiles. 2. When studying the inverse problems of the non-equilibrium atmosphere there is another new class of parameters (denoted by B), describing the processes of excitation and de-excitation of different levels of the molecules – rates of different collisional processes, chemical reactions, etc. These parameters appear in appropriate kinetic equations describing the population of different excited states. 3. The role of boundary conditions becomes more important. For example, solar radiation becomes a source of excited molecules. The boundary conditions in a general case include new quantities, for example, corpuscular radiation. As shown in a number of studies, the measurements of nonequilibrium radiation make it possible to determine the population of excited vibrational levels. This is accompanies by difficulties in determining the population of the ground states of the molecules, i.e. the total concentration of absorbing and emitting molecules. For example, in interpretation of the measurements of the radiometer ISAMS on board the UARS satellite, the content of CO in the conditions of a non-equilibrium atmosphere is determined using a new type of a priori information – a kinetic model of the population of the first excited vibrational state of the CO molecule, – which enables the function of the source to be calculated and tabulated and to solve the inverse problem. This results in a large increase of the volume of a priori information used. In addition to the kinetic model, i.e. the assignment of the physical mechanisms and appropriate constants of the population of excited states, the information on the thermal structure of the troposphere and stratosphere, on the CO content in the lower layers of the atmosphere, on the state of cloud cover, etc. was also used. For a non-equilibrium atmosphere there are several types of inverse problems: 1) in relation to the physical parameters of the atmosphere – a) in accordance with the scheme (see section 10.1) J+A+G → X, 438

(10.5.5)

Radiation as a Source of Information on Optical and Physical Parameters

b)

using kinetic models J+A+G+B → X;

(10.5.6)

2) in respect of the parameters of the processes of excitation and de-excitation – scheme J+A+G+X → B.

(10.5.7)

The two last approaches use the kinetic equations for describing the populations of the excited states. In the first case (scheme (10.5.6)) we determine different parameters of the physical state of the nonequilibrium atmosphere, in the second case (scheme (10.5.7)) – the parameters of the process of excitation and de-excitation of the molecules. This approach was used to determine the profiles of the content of atomic oxygen and hydrogen in the upper layers of the atmosphere from radiation measurements in the 9.6 µm ozone absorption band using appropriate kinetic relationships. The same measurements were used to determine more accurately the important constant of the rate of quenching of excited molecules of CO 2 by atomic oxygen. Thus, in this case, the combined application of the transfer equation and kinetic equations enables us to determine the physical parameters of the atmosphere which do not have a direct effect on the radiation field but strongly effect the population of the excited states of emitting molecules. An example of the retrieval of kinetic temperature of the atmosphere in a wide range of altitudes (40–120 km) including heights at which LTE is not established, is shown in Fig.10.11. These data were obtained using the measurements of the radiation spectrum of the horizon of the Earth in the 15 µm band of CO 2 (apparatus CRISTA) and the interpretation method not using kinetic equations. When interpreting these measurements we also retrieved simultaneously the profile of the kinetic and vibrational temperatures of a number of states of the CO 2 molecule, pressure and CO 2 content.

10.6. Remote measurement methods based on recording the scattered and reflected solar radiation These methods are based on the integral – differential equation of radiation transfer (for example, equation (3.4.35)), and not on its partial cases used previously. The theory and realization of these methods are relatively complicated because it is necessary to take into account the multiple scattering of solar radiation and reflection 439

Altitude, km

Theoretical Fundamentals of Atmospheric Optics

Temperature, K Fig.10.11. Examples of retrieval of the kinetic temperature of the atmosphere in a wide range of altitudes (40–120 km) from measurements of radiation spectra of the horizon of the Earth in the 15 µm band of CO 2 (CRISTA apparatus). 1) 1 – 37º N., 82º W., 2 – 49º S., 172º W., 3 – 19º S., 110º W.

of radiation from the surface. The most frequently used satellite method is the remote determination of the vertical profile and of the total ozone content from measurements of reflected and scattered solar ultraviolet radiation.

Study of the Earth ozonosphere As mentioned in Chapter 4, the values of the absorption coefficient of ozone in the ultraviolet range are very high. For this reason, ultraviolet radiation with the wave length smaller than 290–300 nm does not reach the surface of the Earth. When observing the Earth from space in this range of the spectrum, the solar radiation scattered by different layers of the atmosphere is registered. In the centre of the Hartley absorption band where the absorption coefficients of ozone are very large, scattering takes place in the upper layers of the atmosphere at altitudes of 40–60 km. In the central part of the band where the absorption coefficients of ozone are not very large, the solar radiation penetrates into the thickness of the atmosphere 440

Radiation as a Source of Information on Optical and Physical Parameters

and is scattered at small altitudes of the 20–40 km. Finally, in the wing of the band at still smaller values of the absorption coefficients of ozone (Huggins band) solar radiation reaches the Earth surface – it is reflected by the surface and also scattered by tropospheric layers. Here we have the same effect as the one discussed in the problem of temperature sounding by the measurements of the spectrum of outgoing thermal radiation in the absorption band of carbon dioxide. Measuring the spectral dependence of the outgoing scattered and reflected solar ultraviolet radiation, we carry out ‘vertical’ scanning of the atmosphere. For the quantitative illustration of this class of methods we consider the simplest case. It is assumed that the field of outgoing scattered solar radiation is determined by single molecular scattering in the presence of molecular absorption. This simplified formulation is close to the actual problem of determination of the vertical profile of the ozone content from measurements of scattered solar radiation in the ultraviolet range of the spectrum. For the component of singlescattered solar radiation at angle θ to the nadir we can write

I (λ, θ) = α R P (θ)

∫

F0 0 exp ( −(1 + sec θ S ) × 4π 0 p

1 p  kν′ ( p′)qO3 ( p′)dp′ + τ R ( p )   dp, × g   0 

∫

(10.6.1)

where α R is the Rayleigh scattering coefficient; P(θ) is the Rayleigh scattering phase function; F 0 is the extra-atmospheric radiation flux; θ S is the zenith angle of the Sun; q O3 (p) is the ozone mixing ratio as a function of pressure; τ R (p) is the optical thickness of Rayleigh extinction. The linearization of equation (10.6.1) reduces the examined inverse problem to the Fredholm integral equation of the first kind. Fig.10.12 shows the relative variational derivatives (the kernels of the linearized equation) of this problem characterising the regions of formation of outgoing scattered solar radiation in different spectral ranges of the Hartree–Huggins ozone absorption band. The derivatives are related to the intensity of radiation in the appropriate channel of measurements. Figure 10.12 shows that the outgoing radiation in different spectral intervals forms in different layers of the atmosphere: in spectral ranges with high values of the absorption coefficient of 441

Theoretical Fundamentals of Atmospheric Optics

Altitude, km

nm

nm

Relative derivatives Fig.10.12. Relative variational derivatives (kernels of the linearised equation), characterising the ranges of formation of outgoing scattered solar radiation in different spectral interval of the Hartley–Huggins ozone absorption band.

ozone – in the upper stratosphere, in spectral ranges with small values – in the lower stratosphere and the troposphere. In particular, this special feature of the formation of scattered solar radiation is also a physical basis for the possibility of solving the given inverse problem and obtaining the information on the vertical profiles of the ozone content. This indirect method is used actively for studying the Earth ozonosphere and it makes it possible determine in particular the such the phenomenon as ‘ozone holes’ above the Antarctica. The same principle is also used for studying the content of other gases in the atmosphere – water vapour, methane, etc.

Space survey Many remote methods of studying the atmosphere and surface originated in aerospace methods for military applications. However, it is usually the case, there were other fields of application of spy satellites. For example, earth resources satellites were put into operation. The natural resources include usually a wide complex of the characteristics of the underlying surface relating to geology, water resources, or oceanological, fishing industry, forestry, agriculture, etc. Measuring the outgoing radiation in the ‘transmittance’ windows, i.e. in spectral ranges with small atmospheric extinction, we ‘see’ the underlying surface. As discussed in Chapter 6, different 442

Radiation as a Source of Information on Optical and Physical Parameters

surfaces have different spectral optical characteristics. Thus, it is quite easy to solve the problem of identifying the type of underlying surface. Further, the optical characteristics of the surface are dependent on the state – moisture content, vegetation of different types, and so-on. Using these dependences we can determine, for example, the salt and moisture contents of soil, the condition of vegetation, etc.

Remote refractometry There is also a whole group of inverse problems combined in remote refractometry. To determine different atmospheric parameters we utilize the refraction effects, i.e. distortion of the trajectory of propagation of radiation in the atmosphere as a result of heterogeneities of the refractive index, changes in the phase and the amplitude of electromagnetic radiation. For example, for the microwave range of the spectrum, the refractive index depends on the pressure of the atmosphere, temperature and partial pressure or water vapour. Determining, from space observations, the vertical course of the refractive index and solving the inverse problem using the equation of hydrostatic and the equation of state of the ideal gas, we can formulate and solve the inverse problem of determination of the vertical temperature profile in the stratosphere (where the effect of water vapour on the refractive index is very small) or the inverse problem of determination of the profile of moisture content on the troposphere (for the given temperature profile). Figure 10.13 shows an example of determination of the vertical profile of the atmospheric humidity by this method [104]. In the figure, this profile is compared with the results of independent radiosonde measurements and the results of synoptic analysis of the state of the atmosphere.

10.7. Active remote measurements methods Radar sounding of atmosphere The radar methods of studying the atmosphere were developed as a consequence of using military radars after the First World War. The principle of radiolocation is relatively simple. Pulses of electromagnetic radiation of a specific frequency are sent into the atmosphere and special systems record reflected (back-scattered) signals. The intensity of the reflected signal depends on the distance to the ‘object’ and its properties. Pulse radiolocation

443

Altitude, km

Theoretical Fundamentals of Atmospheric Optics

Humidity, g/kg Fig.10.13. Comparison of the vertical profile of atmospheric humidity, determined by refractometry (1), with the results of independent radiosonde measurements (3–5) and synoptic analysis (2) of the state of the atmosphere [104].

enables easy determination of the distance to the object (because the speed of propagation of electromagnetic radiation is known). The radiolocation methods are most effective is studying clouds and precipitation. They can be used to determine the spatial distribution of clouds and precipitation, the water content of clouds, intensity of precipitation. The physical basis of these methods is the dependence of the coefficients of backscatter of electromagnetic radiation on the number and size of the particles of clouds and precipitation (Chapter 5). At present, there is a large network of meteorological radars, firstly in the system of servicing air transport. In addition, space radiolocation has been intensely developed in the last decade. In this case, the radars are used not only for examining the atmospheric parameters but also the properties of underlying surfaces. Analysing the signal reflected from the surface, we can determine the type and properties of the surface. This principle is also used for such remote methods such as the determination of the velocity of near-water wind and moisture content of soil.

444

Radiation as a Source of Information on Optical and Physical Parameters

Lidar sounding Lidar sounding (LIDAR – Light Detection And Ranging) relates, like radiolocation sounding, to active remote methods of measurements [30, 38, 44]. It may be assumed that lidar sounding is an advanced modification of projector sounding used for examining the atmosphere in the 20s and 30s of the previous century. The physical principles of lidar sounding are identical to radar sounding. The differences are in the wavelengths of the used electromagnetic radiation (visible to long range infrared radiation) and small angle divergence (sharp directionality) of lidar radiation. The lidar sounding methods are based on the equation of laser location which maybe presented in the following simplified form:

∫

 R  E ( R ) = E0 σπ exp  −2 α(r )dr  .    0 

(10.7.1)

where R is the distance to the sounded volume of the atmosphere. The energy of the recorded ‘reflected’ signal E(R) is proportional to the initial energy of radiation E 0 , directed into the atmosphere, the value of the backscattering coefficient σ π (the case in which the source and receiver of radiation are situated at the same point – monostatic measurement scenario) and the value of the transmittance function along the path of propagation of radiation from the source to the sounded volume and backward (multiplier 2 in the exponent – transmittance function P(θ)). It should be mentioned that equation (10.7.1) is written in the approximation of single scattering of lidar radiation. In this case, if the effects of multiple scattering (clouds, mist) are important, it is necessary to modify equation (10.7.1) or solve the problem of the propagation of lidar radiation on the basis of the theory of radiation transfer. Analysis of the equation of lidar sounding shows that the information on the atmosphere is contained in the backscatter coefficient σ π and in the transmittance function P(R). There are different methods of extracting this information which received full coverage in appropriate monographs [38, 44]. It should only be mentioned that at present the lidar method (mostly ground-based ones) provide a large amount of information on different parameters of the atmosphere and the underlying surface. An important special feature of the lidar measurement method is its high information content. This is caused by a number of reasons: 445

Theoretical Fundamentals of Atmospheric Optics

1.Use of different types of lasers of pulse and continuous type in a wide range of the spectrum from ultraviolet to far infrared; 2. Presence of spectrally tuneable lasers of different power; 3. A variety of the process of interaction of laser radiation with the atmosphere – molecular and aerosol absorption, scattering of different types – Rayleigh, aerosol, Raman, etc. 4. Use of different measurement scenarios and different carriers – aircraft, satellites, etc. 5. Short duration and high repetition frequency of laser radiation pulses. 6. The possibility of using laser radiation with specific polarization properties. All these special features make the lidars into a unique tool to examine the atmosphere nd the underlying surface. As an example, we mention the most important parameters of the environment which can be studied using different lidars, and the main mechanisms of interaction of radiation with the medium used for this purpose: 1.Temperature and density of the atmosphere – from the values of molecular (Rayleigh) scattering, Raman scattering, etc. 2. Characteristics of the gas composition – from the values of molecular absorption, Raman and absorption scattering. 3. Characteristics of the aerosols – from the values of aerosol scattering and absorption, polarization characteristics of scattered radiation. 4. The wind field – on the basis of the Doppler shifts of the frequency of scattered radiation. 5. Characteristics of turbulence of the atmosphere – from fluctuations of the characteristics of transmitted and scattered radiation. 6. Different characteristics of the underlying surfaces – from the strength of the reflected signal and its spectral and polarization properties. It should be mentioned that to solve these problems in a general case we can use the measurements of spectral, time, angular and polarisation characteristics of scattered and reflected laser radiation in a wide spectral range from ultraviolet to far infrared.

446

APPENDIX

FUNDAMENTAL UNITS IN ATMOSPHERIC OPTICS AND PHYSICS A.1. Molecular mass of dry and moist air Molecular mass of a mixture of ideal gases In atmospheric physics, the air, like all its components, is assumed to be an ideal gas. Consequently, for the air we can write the equation of state of the ideal gas [19, 68] pV =

m RT , µ

(A.1.1)

where p is the pressure of air; V is the volume of air; m is the mass of air in volume V; µ is the molecular mass of air; T is the temperature of air; R is the universal gas constant. R = 8.31441 J·mol –1 ·K –1 . However, equation (A.1.1) should be regarded as formal because air is a mixture of different gases and, therefore, the concept of its molecular mass requires definition. The ratio m/µ is the number of mols of air in volume V. According to definition, one mol of any substance contains the same number of molecules equal to the Avogadro number N A = 6.0221·10 23 mol –1 Consequently, if N is a number of molecules of air in volume V, then

m N , = µ NA

(A.1.2)

If the mixture contains K gases, the number of molecules of mixture N is equal to the sum of the molecules of each gas 447

Theoretical Fundamentals of Atmospheric Optics

N=

∑N K

i =1

i

. Substituting this equation into (A.1.2) we obtain

∑ K

N m i=1 i = = µ NA

∑µ , K

mi

i =1

i

where m i is the mass of the i-th gas in the volume V; µ i is the molecular mass of the i-th gas; m/µ is the number of moles of the i-th gas. Consequently, µ=

∑ K

i =1

1 . mi 1 m µi

(A.1.3)

Molecular mass of dry air Equation (A.1.3) gives the required definition of the molecular mass of a mixture of gases, in particular, air. In the atmosphere, the composition of the main gases is almost constant (up to an altitude of 90 km), see section 2.1. The variable components are only trace gases but their concentrations (m i /m) are so low that their effect on the molecular mass of air is always ignorable. The only exception is made for the water vapour. The variation of the content of water vapour in the lower troposphere may change the value of µ by several percent. Therefore, as the universal constant we use the molecular mass of dry air µ 0 , i.e the molecular mass of air which does not contain the water vapour [19,68]. It is equal to µ 0 = 28.9645 g·mol –1 . It should be mentioned that µ 0 is constant only up to an altitude of 90 km (see Table 2.2).

Dalton’s law We introduce the partial pressure of a gas in the mixture p i defining it as the pressure of the given gas if at temperature T this gas would occupy the same volume V as the mixture of gases. According to the definition

448

Fundamental Units in Atmospheric Optics and Physics

piV =

mi RT . µi

(A.1.4)

Summing up all these relationship and taking into account (A.1.3) and (A.1.1) we obtain Dalton’s law: the pressure of a gas mixture is equal to the sum of the partial pressures of gases forming the mixture: p=

∑p. K

i =1

i

Molecular mass of moist air From equation (A.1.3) we obtain

m 1 m0 = + w , µ mµ0 mµ w where m 0 is the mass of the fraction of dry air in volume V; m w is the mass of the water vapour in V; µ w is the molecular mass of water vapour. Because of the additivity of the mass m 0 =m–m w we obtain 1  mw = 1− m µ 

from which µ=

 1 mw 1 µ + m µ ,  0 w

µ0 .  mw  µ 0 1+ − 1  m  µw 

(A.1.5)

It should be mentioned that the molecular mass of moist air is always lower than that of dry air (since µ 0/µ w >1) and, consequently, with other conditions being equal, equation (A.1.1) shows that the mass m and the density m/V of moist air are also lower. This means that moist air is lighter than dry air (a paradoxical conclusion because it would appear that the reverse should be the case). The mass of water vapour m w is not suitable for measurements. It is very simple to measure the partial pressure of water vapour denoted by e. Therefore, writing (A.1.4) separately for dry air and water vapour, we obtain a system of equations which is in fact the

449

Theoretical Fundamentals of Atmospheric Optics

equation of state of the system ‘dry air plus water vapour’ [43]:

( p − e)V =

eV =

m − mw RT , µ0

mw RT . µw

(A.1.6)

Separating the first equation by the second one gives

µ p−e  m = − 1 w , e  mw  µo

from which

mw µ p−e =1+ 0 µw e m

or mw e = . m µ0  µ0  p− − 1 e µw  µw 

Usually in calculations the value

(A.1.7)

µ 0 28.9645 = = 1.608 is immediately µ w 18.015

substituted into (A.1.7) and therefore (A.1.7) has the form

mw e . = m 1.608 p − 0.608e Substituting (A.1.7) into (A.1.5) gives the equation for calculating the molecular mass of air on the basis of its pressure and the partial pressure of water vapour:   µ µ = µ0  1 −  1 − w  µo  

e    p

or with the substitution of the value µ w /µ 0

450

(A.1.8)

Fundamental Units in Atmospheric Optics and Physics

 e µ = µ 0  1 − 0.378  . p 

Now, taking into account (A.1.8) we can use for all atmospheric gases, with the exception of water vapour, the equation of state (A.1.1) and all consequences from it. For water vapour, it is necessary to use the equation of state (A.1.6). However, the consequences from (A.1.1) in which there is no dependence on µ, are also valid for the water vapour. In addition to this, if the partial pressure of the water vapour e is low, we can ignore the dependence of µ on e and use in calculations the molecular mass of dry air; here everything depends on the required accuracy of calculations.

A.2. Units of measurement of temperature, air pressure and gas composition of the atmosphere Units of temperature measurements In atmospheric physics, we use the Kelvin and Celsius scales. The relationship between them is: T[K]=T[C]+T 0 , T[C]=T[K]–T 0, where T[K] is the temperature in Kelvin; T[C] is the temperature in degrees of Celsius; T 0 is a universal constant (T 0 is the temperature of absolute zero) whose value is equal to [13, 68] T 0 =273.15K In Great Britain, USA and a number of other countries, the Fahrenheit scale is used but it is found very rarely in the scientific literature. The ratio of the Fahrenheit and Celsius scales 5 T [C ] = (T [ F ] − 32), 9 9 T [ F ] = 32 + T [C ], 5

where T[F] is the temperature in degrees of Fahrenheit.

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Theoretical Fundamentals of Atmospheric Optics

Physical quantities, determining the concentration of gases The concentration of individual gases in the composition of air in atmospheric physics is characterised using not only different units of measurements but also different physical quantities [25]: density, partial pressure, volume, mass and number concentrations. The relationship between them is determined using the equation of state (A.1.1) and may include temperature, pressure and molecular mass of air. Therefore, we shall discuss also the units of measurements of physical quantities determining the concentration of the gas and the relationship between these quantities. In addition to this, taking into account the special equation of state for water vapour (A.1.6), for H 2 O there are special measurements units.

Pressure of air and partial pressure of gas The main unit of pressure used in atmospheric physics is millibar (mbar) or hectapascal (GPa). the derivatives of the millibar, used usually for measuring the partial pressures of trace gases, are: pascal (Pa), i.e. (N·m -2 ), millipascal (mPa), micropascal (µPa). An important constant in atmospheric physics is the physical atmosphere or bar [19, 68], equal to p 0 =1013.23 mbar. In Russia, the unit of measurements of atmospheric pressure is the millimetre of the mercury column (mm Hg) for which p 0 =760 mm Hg, which determines its relationship with the milllibar. A chain for converting the units of measurement of (partial) pressure is: p[mbar]=1013.25 p[bar]=1.33322 p[mm Hg]= =0.01 p[Pa]=10 –5 p[mPa]=10 –8 p[µPa], where p[…] is the value of pressure in appropriate units.

Partial density The partial density of a gas ρ i is its density in the conventional meaning of the word, i.e. the mass of the gas in the unit volume of air. Dividing both parts of (A.1.4) by V we have

ρi =

pi µi , RT

452

(A.2.1)

Fundamental Units in Atmospheric Optics and Physics

which gives the relationship of partial density and partial pressure at the temperature of air T. Equation (A.2.1) also holds for the density of air as a whole if the quantities in this equation are written without the index i. The set of the units of partial density varies greatly depending on the selection of the mass and volume units. In atmospheric physics gram per cubic meter is used in most cases (g·m –3 ). The chain for converting the units of measurement of partial density is: ρ[g·m –3 ]=10 6 ρ[g·cm –3 ]=10 –3 ρ[mg·m –3 ]=10 –6 ρ[µkg·m –3 ], where ρ[…] is density in appropriate units. When substituting the quantities in specific units of measurements, equation (A.2.1) has the form ρ [g ⋅ m −3 ] = 100

pi [mbar] µi [r ⋅ mol−1 ] . R [ J ⋅ mol−1 ⋅ K −1 ]T [ K ]

Volume (molar) concentration (fraction by volume) The volume concentration of a gas q i is the ratio of the partial volume V i – the volume, which the gas would occupy under temperature and pressure of the gas mixture (air), – to the volume of the gas mixture (air). In some cases the volume concentration is also referred to as the volume mixing ratio. According to the definitions of partial volume

pVi =

mi RT . µi

(A.2.2)

Dividing both parts by V, we obtain the relationship of volume concentration and partial density

qi =

ρi RT µi p

(A.2.3)

at the given temperature T and air pressure p. Expressing ρ i from (A.2.1), we obtain a simple relationship of volume concentration with partial pressure

qi =

pi . p

453

(A.2.4)

Theoretical Fundamentals of Atmospheric Optics

The molar concentration of a gas is the ratio of the number of molecules of the gas in the unit volume to the number of molecules in the gas mixture (air). The number of molecules of the gas is m i / µ i , for air – m/µ, and the division of (A.1.4) by (A.1.1.) gives the molar concentration p i /p, but this according to (A.2.4) is q i . This means that the molar and volume concentrations of the gas are equal. It should be mentioned that this holds only for the ideal gas but, for example, not for a mixture of liquids. Volume concentration is a dimensionless quantity but this does not mean that it has no measurement unit: these units are the fractions of the volume. The main unit of the volume concentration of the gases in atmospheric physics is the millionth fraction in respect of volume – mln –1 (ppm V, sometimes written simply as ppm, indicating that we are discussing volume fractions). Other widely used units are: the volume mixing ratio, i.e. the direct (without conversion) fraction of the volume (vmr); volume percent (vol%); volume promille – prom; the billionth part by volume (ppbV), the trillionth fraction by volume (pptV). The conversion chain of the units of measurement of volume concentration is: q[ppmV]=10 6 q[vmr]=10 4 q[vol%]=10 3 q[prom]= =10 -3 q[ppbV]=10 -6 q[pptV], where q[…] is the concentration in appropriate units. When substituting the values in specific units of measurement, equation (A.2.3) has the form

qi [mln −1 ] = 104

ρi [g ⋅ m −3 ] R[J ⋅ mol−1 ⋅ K −1 ]T [ K ] . µi [g ⋅ mol −1 ] p[mbar]

Mass concentration (fraction by mass) The mass fraction (concentration) of the gas w i is the ratio of the mass of the given gas to the mass of the gas mixture (air) in the given volume. Dividing (A.1.4) by (A.1.1) gives wi =

µ i pi µ p

and

taking into account (A.2.4) the simple relationship of the mass fraction and volume concentration, we have: wi =

µi qi . µ

454

(A.2.5)

Fundamental Units in Atmospheric Optics and Physics

It should be mentioned that equation (A.2.5) cannot be used for water vapour because the value of µ for the latter depends on w i . Equation (A.2.5) shows that for the gases in which the molecular mass is higher than the mass of air, the mass concentration is larger than volume concentration; for gases in which the molecular mass is smaller than the mass of air, the mass concentration is also smaller than volume concentration. The relationship of the mass fraction with partial pressure and density of the given temperature and pressure of air, according to (A.2.3)–(A.2.5) is

wi =

µi pi ρi RT = . µ p µ p

(A.2.6)

Mass concentration is a dimensionless quantity and is expressed by different fractions of the ratio. The main unit of the mass fraction in atmospheric physics is gram per gram, g/g (kilogram per kilogram, and so on). This unit shows how many grams of gas are present in the gram of air. Other units include: gram per kilogram, g/kg (milligram per gram mg/g, promille by mass – prom); per cent by mass – %; milligram per kilogram, mg/kg (millionth fraction by mass – mln –1 (ppm); (microgram per kilogram µg/kg (ppb)) the trillionth fraction by mass, (ppt). The chain of conversion of the units of measurement of the mass fraction of the gases: w[g/g]=10 –3 w[g/kg]=10 –2 w[%]=10 6 w[mg/kg]= =10 –9 w[µg/kg] = 10 –12 w[ppt –1 ], where w[…] is the fraction in the appropriate units. When substituting the quantities in specific measurement units, the second of equations (A.2.6) has the form: wi [g / g] = 10−2

ρi [g ⋅ m −3 ] R[J ⋅ mol−1 ⋅ K −1 ]T [ K ] . p[mbar] µ[g ⋅ mol −1 ]

Number concentration The number concentration of a gas n i is the number of molecules of the gas in the unit volume. Writing (A.1.4) through the number of gas molecules N i and dividing by volume, according to the definition we obtain n i =p i N A /RT. The constant k B =R/N A is the Boltzmann constant – one of the fundamental physical constants [19, 68] 455

Theoretical Fundamentals of Atmospheric Optics

k B =1.380662·10 –23 J·K –1 . Finally we obtain the relationship of n i with the partial pressure of the gas and the temperature of air:

ni =

pi . k BT

(A.2.7)

Formula (A.2.7) can be used for air if we set p i =p. Taking into account (A.2.7) from (A.2.1), (A.2.4), (A.2.6) we obtain the relationship of the number concentration with the partial density, volume and mass concentrations at the given temperature T and pressure P of air

ni = N A

ρi µ p p , ni = qi , ni = wi . µi µ i k BT k BT

(A.2.8)

The third of the equations (A.2.8) cannot be used for water vapour. It should be mentioned that number concentration is important in atmospheric optics because it is essential for calculating the volume coefficient of molecular absorption from the available absorption cross-section of the molecule (see section 3.3). Therefore, in optics it is often simply referred to as ‘concentration’ without the word ‘number ’. The measurement unit for number concentration is represented in most cases by the inverse cubic centimetre (cm -3 ), i.e. concentration is expressed by the number of particles in the cubic centimetre. Other units are usually not used for number concentration. When substituting the quantities in specific measurement units, equation (A.2.7) and (A.2.8) have the forms

ni [cm −3 ] = 10−4

pi [mbar] , kB [J·K −1 ]T [ K ]

ni [cm −3 ] = 10−6 N A [mol−1 ] ni [cm −3 ] = 10−10 qi [ppm V] ni [cm −3 ] = 10−4 wi [g/g] 456

ρi [g·m −3 ] , µi [g·mol−1 ] (A.2.9).

p[mbar] , k B [J ⋅ K −1 ]T [ K ]

µ p[mbar] . µi k B [J ⋅ K −1 ]T [ K ]

Fundamental Units in Atmospheric Optics and Physics

A.3. Units of measurement of the concentration of water vapour Specific forms of expressing the concentration of water vapour For the concentration of water vapour, we use both the previously mentioned five types of units (partial pressure and density, volume, mass and number concentrations) and also additional specific types of units [25,43].

Partial pressure of water vapour Using the partial pressure of H 2 O denoted by e, we can express other concentrations of H 2 O. In some cases e is also referred to as the elasticity of water vapour. The units of measurements of this parameter are the conventional units of partial pressure (see A.2).

Partial density of water vapour Absolute humidity of air The partial density of water vapour is determined using the same procedure as the conventional partial density (see A.2). It is described by the relationship (A.2.1) in which p i =e. The synonym of the partial density of water vapour is the absolute humidity of air. Absolute humidity is the mass of liquid water which could be expressed from the unit volume of air. The standard unit of absolute humidity a is g·m –3 . In substitution µ i into (A.2.1) for water we obtain a[g·m −3 ] = 217

e[mbar] . T[K ]

(A.3.1)

Volume concentration of water vapour This concentration does not differ at all from the volume concentration of other gases and is expressed in the same units. From (A.2.4) we have

e qi = . p

(A.3.2)

Mass fraction of water vapour The mixing ratio of of water vapour In old terminology, the mass fraction of water vapour is often referred to as the specific humidity of air [43]. Its definition and 457

Theoretical Fundamentals of Atmospheric Optics

the units of measurement are identical with the mass fraction of any gas but the equations for expressing it should be corrected taking into account the dependence of the molecular mass of air on the value e. The formula of the relationship of w =

mw , where m w is in g/g, with the partial pressure is (A.1.7). This equation gives the required expression for the partial pressure of water vapour through its mass fraction at the given pressure of air p: µ0 w µw e= p .  µ0  1 +  − 1 w  µw 

(A.3.3)

With numerical substitutions e= p

1.608w[g/g] . 1 + 0.608w[g/g]

Using (A.3.3) to express e through w from (A.2.1) (or (A.3.1)) and (A.2.4) or (A.3.2), we obtain the relationship of partial density and the volume concentration of the water vapour with its mass fraction. Taking into account numerical substitutions we obtain a[g·m −3 ] = 349

w[g/g] p[mbar] , 1 + 0.608w[g/g] T [ K ]

qi [ppm] = 1.608 ⋅106

w[g/g] . 1 + 0.608w[g/g]

In addition to the conventional definition of the mass fraction of water vapour we introduce also the mixing ratio s – the ratio of the mass of water vapour in the unit volume to the mass of dry air in the same volume. According to the definition s = m w /m 0 =m w /(m– m w), from which by dividing by m we obtain the relationship of the ratio of the mixture and the mass fraction of water vapour

s=

w s ,w = , 1− w 1+ s

where w, s are in g/g.

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Fundamental Units in Atmospheric Optics and Physics

Number concentration of water vapour This concentration is defined as the number concentration of any gas and is measured in cm –3 . The relationship of the number concentration with partial pressure, density and volume concentration is given by equation (A.2.7) and the two first equations of (A.2.8). To determine the expression for the number concentration through the mass fraction of the water vapour we substitute (A.3.3) into (A.2.7) and, taking into account numerical values, we obtain n [cm −3 ] = 1.165 ⋅1019

p[mbar] w[g/g] . T [K] 1 + 0.608w[g/g]

Relative humidity of air The partial pressure of water vapour cannot be higher than some value – saturation pressure E which depends on the temperature of air, T, and as T increases the value of E also increases. For the function E(T) we have theoretical expressions but they all were derived using some approximations and are fulfilled with insufficient accuracy. Therefore, for E(T) it is necessary to use usually semiempirical relationships obtained by correction of the theoretical equations for better agreement with the experimental data. In particular, the WMO (World Meteorological Organisation) recommends the equations [69, 70] lg (E)=10.79574(1–T 0 /T) –5.028lg (T/T 0 ) + +0.42873·10 –3 (10 4 76955(T/T 0 –1) –1)+ 0.78614,

at T ≥ T 0 ,

lg (E) = –9.09685(T 0 /T–1)–3.56654 lg (T 0 /T) + +0.87682(1 – T/T 0 ) + 0.78614,

(A.3.4)

at T < T 0 .

where T is the temperature of air in K; E is the saturation pressure, mbar; T 0 = 273 K. The relative humidity of air (u) is the ratio of the partial pressure of water vapour to saturation pressure: u=

e . E (T )

(A.3.5)

The meaning of relative humidity is the capacity of liquid water for evaporation: as the value of u increases the rate of this process 459

Theoretical Fundamentals of Atmospheric Optics

decreases, at u close to unity evaporation cannot take place and the inverse process of condensation of water from the atmosphere (dew and frost phenomena, formation of fog and cloud) starts. The relationship of the relative humidity with other units of the concentration of water vapour can be easily determined expressing partial pressure e from (A.3.5) e=uE(T) at the given temperature of air. The relative humidity of air is expressed in percent and, less frequently, in fractions. The characteristic of the concentration of water vapour, similar to the relative humidity is also the deficit of pressure – the difference between saturation pressure and the partial pressure of water vapour E(T) – e.

Dew point Dew point t is the temperature at which the partial pressure of the water vapour present in the atmosphere becomes equal to saturation pressure, this means that e = E(t).

(A.3.6)

It should be mentioned that the dew point is the unit of the concentration of water vapour, although it is expressed in temperature. To determine other concentration units from the known value of the dew point it is sufficient to use the partial pressure of water vapour determined from (A.3.6). To determine the dew point from other units it is necessary to find the value of e and then use the function t = E –1 (e) inverse to (A.3.4). The inverse function T(E) exists because of the monotonicity of E(T). Equation E(t) = e is easily solved in a computer (for example, by the halving method). The dew point is usually measured in degrees Centigrade. The dew point deficit – the difference between the temperatures of air and the dew point T–t is also examined. In night cooling of the Earth’s surface and air (see paragraph 2.3) when the temperature decreases below the dew point, the relative humidity of the air reaches 100% and water vapour condenses. If the dew point temperature is higher than 0 o C, dew appears, if t